Tiny sentience runs into the problem of brain size. While human medicine doesn't know everything about the brain, we know enough to identify regions that do certain tasks. Below a given threshold, there isn't enough space for all the regions, so with human physiology, it seems clear tiny humans would have to give up functionality.
But... do neurons have to be so large? Transistors of past decades are giant compared to the ones we use in computers today. Maybe neurons can be implemented smaller? (I am dodging the question of whether transistors are sufficient to replicate human intelligence... that's a topic for another day/forum.) Evolution has never to my knowledge had a reason to select for smaller neurons in humans. Maybe they can be made more space efficient without loss of functionality. That's an open question for science, so far as I know, so you're free to posit what you like.
Alternatively, if the braincase gets smaller, you might get different brain structure. Perhaps the limbic system stretches down the spine. Or the thyroid is near the gut. By distributing pieces, you might free up room in a tiny skull. From the outside, they'd still look human. Downside: injuries that to us are healable might not be to them. "Oh, a bolt through the arm. We can stop the blood, but that's where his autonomic systems are controlled from. I'm afraid he forgot how to breathe." That sort of thing. The torso/skull's advantage is it gives us one "critical region" to protect, and we can afford loss of limbs. The more the limbs have critical infrastructure, the worse it gets.
So, yes, there are ways to have tiny sentients that seem human. Just don't look inside or expect human doctors to treat them!
