Pretty sure we're at "speck of flour".
This is probably around the smallest scale we can be reasonably certain exist in D&D. This includes the 10 µm width of cotton fibres. We can't be completely certain that it's made of fibres in D&D, though. Smaller than that, spider web silk is 3-8 µm in our world, but well, Lolth might have a profound impact in what the web sizes are for the small spiders we're talking about for those webs..
Flour particles can be that small, but I mentioned the median size earlier because around 95% of it is 50 µm or larger, which means you won't really be find the smallest speck.
hmm ... I wonder about the Grinding techniques
used on the Flour. Obviously a well ground speck
of flour exists, but the size of it won't be what modern
food engineering does. I doubt Dragon mag or any
official d&d Novels had anything on grinding wheat.
I could see this becoming quite a debate about how
small something is, that doesn't,
can't, and "shouldn't"
use exact real world measurements.
Smallest d&d measured spider is that 1" on post #5.
idk --- 1" real life spider has what size of silk ??
If you can see the silk, it's bigger than the Cerebral Parasites.
Spores ?!
Seems to require a real life assumption, but still.