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Without resorting to solid osmium inner cores that we couldn't give an origin story for, what's a cosmologically plausible lower bound of a planet's radius that also maintains 1G gravity and the sort of mantle conditions you need for plate tectonics?

MaximalDestrier
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Using the same methodology I used in my answer to How Big Can a Habitable Planet Get Before Its Gravity is More Than 0.8 m/s² above Earth's 9.81 m/s²? I've made the following table. The last line is data for Earth.

https://i.stack.imgur.com/pVCRo.png

A planet that has a radius that is 0.62 the Earth radius and 0.3836 the mass of the Earth will have a surface gravity very close to $g$ and an average density of 8.876 t/m3 (g/cm3). This density is very close to the density of Kepler-10b.

All that is needed for plate tectonics are significant oceans above the subducting plates so water can lubricate the plates during subduction.

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    dont forget that plates also require an active core thats hot enough and moving around often enough to keep the plates from settling too soon – zackit Apr 13 '21 at 16:23
  • What I was thinking would be an issue for plate tectonics is the requirements for mantle fluidity. Otherwise you can just stipulate a relatively pure iron planet (or hell, even a chthonian. It takes quite a while for those to bounce back to hydrostatic equilibrium). Would that not be an issue? – MaximalDestrier Apr 14 '21 at 17:24
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    @MaximalDestrier: You are correct. –  Apr 15 '21 at 05:28