I mean, I'm pretty sure, and as near as I can tell from a quick refresher google correct enough for idle chatter, there actually
are words for the smallest segments of linguistic or written expression (which is somewhat separate from the axiom babble, to be fair)? The phon/morph/graph -eme stuff (word sound, concept, physically written thingy, more or less and probably scrubbing over nuance 'cause, again, I'm just not terribly interested in the subject), which are probably broken down even more by folks that really bury themselves in that sort of thing.
The axiom thing theoretically
can be big, but they're also where the buck stops in regards to (formal, in particular) logic and you stop being
able to break down a concept or proposition any further 'cause there's just nothing left you can produce but what you assume. They're the building blocks of everything we do in terms of anything systemic (i.e. All communication and thought, at a minimum), literally the only
possible indivisible aspect of a concept, and consequently the tiniest you can chop an idea down to

Though it might be worth eternally noting that smallest or simplest doesn't always entail small or simple, heh.