The Atom can reduce the size of his own body with a Class 5000 ability. In practicle terms, he can enter sub-atomic universes. His preferred fighting size is about 6 inches.
The Justice League of America
As a graduate student, Ray Palmer was trying to find a way to reduce matter in an effort to aid farmers in increasing their yield. With each attempt, the matter was reduced by a beam but quickly exploded. From the fragment of a white dwarf star, Palmer found the missing properties needed to alter the beam to a reducing lens.
Regardless of the white dwarf matter, objects continued to remain unstable after reduction. Sometime later, when trapped in a cave-in with a group of youngsters, Palmer tried the lens on himself, risking death to free the others. After Palmer saved the group and did not explode, he investigated further and found that a combination of ultra-violet rays, cave water, and something in his own physiology allowed him to safely reduce his size.
Palmer developed hand controls that permitted him to alter his size and weight at will. Using some of the star matter, he fashioned a costume that appeared only when he was at a size less than his normal six foot height.
Palmer graduated from college and became a physics professor at Ivy University. As the Atom, he battled crime to help his ladylove, Jean Loring, establish herself as a lawyer so she would agree to marry him.