In my world I am working on, the species is similar to humans in almost every way except for the following thing things:
What evolutionary history would support this form of human-like creatures?
You should also have included an introduction to the physics of your world. As in, the type of planet (if it is other than earth), it's atmospheric composition (especially oxygen level), the characteristics of other creatures in the world (are they all lean and large or only humans?) and anything else you have placed differently from earth.
In my answer I am assuming these lean characteristics are limited to humans of the world and other species are as heavy as their counterparts are on earth.
Hair Color
It would have helped if you also stated WHICH hair color was the norm for your people. Anyhow, for generally "light" colored hair, you can use diet preferences. For example, your human(oid) creatures have been eating cereals x, y and z for millenia. These cereals have large quantities of zinc/iron/copper salts in them, which gradually found their way in the hair coloring protein. Similarly if they are also a carnivorous peoples, you can add the same metal salts mention for their meat diets to explain their hair color. Only now those salts are also present in the meats of the animals they hunt and consume. You can also make an interesting grouping of your people by hair color too. People who eat more mutton than vegetables have light brown color hair. People who eat more vegetables have rather grayish hair. People whose diet mainly consists of fish ...
Here I am assuming that by "light hair", you are implying light-colored hair. If you intend to mean "less hair" or "thin hair", please state so clearly.
Height
This can be a result of:
a) Sexual Selection. During the early stages of these human(oid) evolution, taller males were preferred by the females. Taller males having more access to females than shorter males would mean more babies born of taller males which would increase the chance that a taller baby would be born. Repeated over millenia, the general height of these people came out to be 7 feet which is the tallest height supported by their skeletal structure. People can grow taller than that, but they will have problems walking and daily life activities so they are not much preferred by the females. Hence the general height of the individuals is set at 7 feet.
Now comes the very important question of WHY the females preferred tall males? One answer can be that the region where early human(oid)s lived, was grassland area and the grass was really tall. A tall male was more likely to see above this green layer. His and his mate's life would be somewhat easier than a short male, hence females preferred taller ones.
Or/And you can specify that the trees in the area of early evolution stages had their lowest branches sprouting at quite some height. So in times of danger a tall male had better chances of jumping and grabbing a tree branch to save himself and later pull his mate on the tree too.
b) Survival Selection. As in, tall males were more likely to survive in the early stages of these human(oid)s and hence had more chances to reproduce. The difference here is that females don't have a mate preference based on height of males, but taller males are more likely to survive while shorter males died before reaching reproduction age.
Once again you are going to have to throw in reasons of how/why taller males were more likely to survive where shorter males perished.
Were taller males better suited for hunting? Foraging? Was their ability to see for longer distances a key factor?
Slenderness
This is more easier accounted for, than height. We can use two approaches here.
a) Diet. The plants/animals consumed by the human(oid)s contain very little phosphorus. This lack of phosphorus would imply slender, lightweight bones. Lightweight skeleton leads to a thinner muscles and a generally lean build.
You can also use a lack of calcium in their foods but I would rather not prefer that. Calcium is also a key constituent of teeth and you might not want to give them weaker teeth.
b) Survival/Sexual Preference. Lightweight individuals run faster than heavy ones and this can be the key difference between getting caught by a chasing predator and outrunning it till you reach a shelter. Or, for reasons lame and unknown (known only to women), the females could prefer skinnish males over heavier ones.
Longer, Prehensile Toes
This can be easily explained IF your peoples are in early stages of leaving the jungles and settling on the ground. Or maybe they still have to climb trees often for one reason or another. OR maybe they have this super ability to pick up objects with their feet as well as hands, which makes them far more successful engineers/craftsmen than others who can only use their hands for picking up objects.
This species comes from a warmer climate on a smaller earth The changes to normal human physiology aren't that big so it shouldn't be too bad.
They a much more slender - Allen's Rule describes the relationship between surface area to internal volume depending on prevailing climate. Taller, more slender builds tend to work well in warmer climates. Shorter, stockier builds work better in colder climates.
They have very light hair - In humans, lighter hair is correlated with decreased exposure to sunlight. Note the blonde hair of the Scandinavians compared to the dark hair of native Africans.
They are 7 feet tall on average - Giraffes are so tall because they are under evolutionary pressure to reach taller branches that other animals can't reach. So, the species could be under similar pressures to reach higher food sources. Alternatively, spider monkeys have very long arms that they use to swing through the trees.
Their toes are longer and better at gripping - If the species is modeled on spider monkeys then extra grippy toes shouldn't be too hard.
Numbers 1,3, and 4 could all be the result of a vertical environment, whether that was cliff faces, jungle, or arcology ruins. To be an evolutionary driver, the environment would need to be the primary living area. So possibly a "Jungle World" or the like.
Number 2 could go along with living permanently under a jungle canopy.
Alternately, have you ever read Larry Niven's Integral Trees and Smoke Ring? Zero G living might also lead to such traits.
Initially evolving in a Zero G world would be unlikely, but perhaps a Generation ship with larges open areas of either non-spin or slow-spin, or a caste purposed with maintaining the non-spin and external portions of the ship could reinforce those traits.
As you state "the species is similar to humans in almost every way" aside from the 4 physical exceptions I'm going to assume they'd be anatomically analogous to really tall, slender humans. At least the most general terms
I'm not sure if your tall humanoids would be living in forest/jungles based on human variation. Humans living on the African savanna are taller on average than jungle dwellers. My guess is that thick undergrowth and limited visibility in jungles don't favour being tall in the same way it might for a species living primarily on the plains.
One idea that could work is that their planet is a world mainly composed of temperate plains - at a high latitude meaning less sunlight like in Northern Europe which has a lot of fair-haired people - the plains could be broken up by small forested areas that have very tall trees with calorie-rich fruit. A tall, slender humanoid with good grip on their feet would be able to make the nomad circuit between the oases and reach the fruit once there. This might be putting the cart before the horse in terms of evolutionary cause and effect but this stuff can be difficult.
If you want your slender humanoids' upper height limit to be around 8 feet (give or take) then they likely need their home planet to have lower gravity relative to Earth's. Probably similar to Mars' gravity well - which is about 40% of Earth's.
Perhaps they'd have a longer lifespan than humans since larger animals often have a longer period of adolescence during which they reach their full adult size. Kind of like great apes or elephants. It could be that they had a longer life span due the fact that they don't reach reproductive age until the human equivalent of their mid-twenties.
Link to reference article mentioning longer development periods for larger animals - http://www.smithsonianmag.com/science-nature/how-death-played-a-role-in-the-evolution-of-human-height-152527401/?no-ist
It's simple, they live in a hot environment. You can say the light colored coat reflects more heat. You can also say their planet is of lower gravity. Lower gravity endorses more height and less volume. You can also say there was an evolutionary preassure to be taller since they eat fruit in trees just like Giraffe's. Additionally more height and less volume makes it easier to disperse heat, so you can stick to the hot planet idea.
Source: https://www.health.harvard.edu/staying-healthy/out-in-the-cold
Body type explains some of the varying reactions to cold weather. Taller people tend to get cold faster than shorter people because a larger surface area means more heat loss. And fat's reputation as an insulating material is well deserved, although for warmth during the winter, you want it to be the subcutaneous fat layered under the skin, not the visceral fat that collects in the abdomen.
