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Using a gaseous form of "Pym particles" kept in a compartment in his belt, Ant-Man initially had the power to shrink himself (and other people and objects along with himself) to the size of an ant and return to normal. Over time, he has acquired the ability to change size at will.
Consequently, is Ant Man stronger when small?
Whenever Scott shrinks down, whether it'd be ant sized or subatomic, he keeps the same strength as he had normal human being sized. That's why as demonstrated in his movie or during Civil War, Ant-Man was pretty strong despite his size. However, his strength only is portrayed whenever he's small.
Beside this, how small can ant man go?
We then see him shrink to the size of an atom, even then he is not small enough, but he keeps going…he even gets smaller than quarks – which are particles we haven't even properly identified. Scott at this point is really, really small – as small as 10/-32mm, which according to his mass is enough to make a black hole.
The Ant-Man suit uses what Marvel calls Pym Particles (subatomic particles that are capable of adding or reducing mass or scale). Basically, they can grow or shrink an object in size and increase the density and strength of the subject (which is why the suit increases the wearer's strength while they change size).