Too tall.

exactly lap sized

How big can a knickknack make themself? And how small? Is there a limit?

you can be basically as small as a finger and as big as a whale

after a certain point of size you can't get material that fine to work with, or you're too big to move under your own power/can't acquire enough mats

At those wayy too big sizes, it might be more energy efficient to move around like a giant amoeba, just building yourself in whatever direction you want to move. Snip off your butt, slide it through your innards, and vomit up enough fabric to occupy the place you want to be.

Also if they ever invent microscopes, do you think they might be able to weave microscopic bodies out of long protein stands? Or carbon nanotubes? But then what would they use as a needle...

you're making the assumption that the world is built out of molecules

Hol up, does that mean that they aren't limited by the size of atoms? That they could theoretically go +even smaller+, given they find an appropriate material?

Then it's all just a matter of finding a way to manipulate such materials, or filling down larger things into even microscopic-er threads. If the size of their eyes defines their field of view, one could glue a tiny mote of dust to their body, hollow out a few hairs to use as arms, and use that to work on an even tinier body. Then, with that tiny critter, build one even tinier, and so on until they're too small for the narrative to track, or they get bored.

I imagine they can't get any smaller then the buttons you would think.

you can use other things for eyes, buttons are just traditional because it's what Mom Always Used™