I suppose you're asking about the theoretical limit of MicroSD rather than the theoretical limit in general, but there's never a bad time to read Feynman's essa

I suppose you're asking about the theoretical limit of MicroSD rather than the theoretical limit in general, but there's never a bad time to read Feynman's essay, "There's plenty of room at the bottom"! https://web.pa.msu.edu/people/yang/RFeynman_plentySpace.pdf https://web.pa.msu.edu/people/yang/RFeynman_plentySpace.pdf