[ZirconiumX]

A more complex instruction set needs more silicon to decode, and thus is less power efficient. However, I'd imagine that Intel are pretty good at decoding x86 at this point; they've had plenty of practice, so it might balance out.

[astrodust]

A 7nm process Pentium III is going to use almost no power and take up very little space. An 80486 would be even tinier.

[tracker1]

Could even be a modernized 386 with cache and integrated co, effectively a 486dx without all the additional instruction support.

[flamedoge]

yes, but usually complex = variable length encoding. size is also a factor.