A Silithar particle is "no more than 1/2 inch diameter"
A_D_M_G already referenced the relevant part of the parasites.
Even if I assume a sling bullet is the same size as a real world sling-ammo marble (.68 caliber, or .68 inches diameter) you get something the same size as the Silithar particle.
So far, the microbe is the smallest.
What spawned this question:
Atoms don't exist by D&D physics, and there's no size category smaller than Fine.
Matter is made of atoms, what else should it be made of?
Note that the Fine size category is only defined as "6 inches or less". Note also that it only pertains to creatures.
Matter in D&D is made of the prime elements, plus either negative or positive energy. Four elements, two energies.
Earth, Air, Fire, and Water each comes in a multitude of variants, and coalesce each in their own way.
A 50lb granite rock in D&D is not 20-trillion atoms in just the right configuration. It is 50lbs of Earth element of the granite variety.
Support for what you claim:Despite the fact that weird shit happens, physics operate normally
Support for what I (and Prime) claim:Within the D&D cosmology, four basic elements and two types of energy together make up everything. The elements are earth, air, fire, and water. The types of energy are positive and negative.
The Material Plane reflects a balancing of those elements and energies; all are found there.
Note: heavily redacted from the original source in order to leave only pertinent data.
So I was curious, given my granted assumption / interpretation, what would be the smallest thing in D&D.
Even going so far as steam
not being tiny droplets of water, but rather its own variant of material combining Water and Air elements.