A is also for Atom

Professor Ray Palmer is the ATOM, an expert physicist at the prestigious Boston University.

The Atom possesses a belt that allows him to shrink his body to varying degrees, including the subatomic level. This ability is not restricted to him, as he is also capable of reducing the size of other people of objects he is in contact with. However, whilst he has shrunken himself, he retains all of his physical strength that he would have at his normal size, making him seem surprisingly strong.

The Atom (Ray Palmer) and was created by Gardner Fox and Gil Kane, and first featured in Showcase #34 (Sep-Oct 1961). Raymond Palmer was a physicist and university professor who devised a reducing ray using a fragment of white dwarf star matter.

Ray Palmer discovered that he could safely use the beam on himself, complete with being able to easily resume his normal size, without any side effect. Palmer then adapted his shrinking technology to a costume, and used it to fight crime as The Atom.

Palmer later developed a new technology to attain desired molecular densities, which enabled him to travel through phone lines and air currents, and throw a punch carrying the same impact of his normal size, while on subatomic level.

Marvel Comic's equivalent of a small size superhero? Ant-Man!
Ant-man is the name of several fictional characters in the Marvel Comics Universe. Ant-man was originally the superhero persona of Hank Pym, a brilliant scientist who invented a substance that allowed him to change his size. The character was created by Stan Lee and Jack Kirby and first appeared in Tales to Astonish #35.

After Pym retired his Ant-Man identity, successors Scott Lang and Eric O'Grady have used Pym's technology to take on the role of Ant-Man.