Not Really
There are a couple of creatures that are small and rather clever. I remember stories about octopuses for example escaping from their tanks, and one the says the octopus snuck into another thing to eat some of the fish and then snuck back into its own tank. I even remember the story about a guy who was training a raven to use a keyboard only for that raven to sneak out of it enclosure, destroy the keyboard, and sneak back into its enclosure in order to play innocent. While it is true that bigger size might allow more room for a bigger brain, never underestimate the cleverness of the little guy.
Microorganisms might have to work together though
I think for a microscopic organism you’re going to have to have a lot of them working together as a colony in order to become sentient. There’s an interesting species from a lovely Stargate episode called Message in a Bottle to give you an idea of what I’m talking about. https://stargate.fandom.com/wiki/A%27t%27trr