31x4 = 124 bytes :-) The Zero register doesn't have to physically exist, and we're talking about 32 bit CPUs e.g. Pi Pico 2 here, right.

Ok, 128 bytes if you add in the PC.

Except for RV32E -- as seen in the very popular $0.10 CH32V003 -- which has 15x4 = 60 bytes of GPRs, plus the PC.

Plus usually a few CSRs on practical CPUs, though Zicsr is an extension so you don't have to have it.