I remember a Flash Gordon
story, drawn by Gil Kane, where he had been reduced to sub-atom size and found himself on a planet - and monsters.
E.C. (comics) undoubtedly explored the proposition more than once. The Atom
series, I think, did something similar.
I suspect, though I only have a vague recollection, that this was included in the premise behind Micronauts
I remember having the discussion with the family after (my) first viewing of The Incredible Shrinking Man
- at what point would he stop shrinking - so the idea can't have been entirely original.
It's an idea with absolutely no scientific basis, of course (he says smugly, but without proof), as the short-hand descriptions we were given at school of electrons and protons orbiting a nucleus, like moons of a planet, were necessarily over-simplifications of their actual nature of layered energy fields (I hope the real scientists will tidy up my necessarily over-simplified stab at a description
I think that, now we're becoming more familiar with the huge possibilities offered by our own solar system, by the galaxy in which it resides, by the neighbourhood our galaxy occupies and the Universe it's all encapsulated by, the possibility of sub-atomic life might be explored properly, but it's unlikely, even in fiction, to resemble the structure of space as we see it.