We can see things that are about 0.1 millimeters long … which is about the thickness of thread.
If you have good vision you should be able to see things of that size without glasses, but you’ll likely only notice them if they are black and on top of a white piece of paper, or white and on top of a black background.
Actually it is possible to see things that are even smaller than that!
Have you ever sat inside on a sunny day with the curtains partly closed? The beam of sunlight that shines into the room will often illuminate dust particles. Dust particles can be made of pollen, human skin, soil, fibres, soot and moulds. The average dust particle is between 2 and 4um. This is equal to 0.002 to 0.004mm. So your eyes are pretty good at seeing very small things. Anything smaller than this would probably require a magnifying glass or microscope.