I remember this character from a superhero comic book. It was almost certainly a book by a US publisher, and I'm pretty sure it was not older than late 90s.
I'm also pretty sure that the character in question is male. His super power is that he can change his size at will. The interesting twist here was that his intelligence is directly tied to his size: At normal size he is of average intelligence, but the more he shrinks, the smarter he gets, and conversely, he gets dumber as he grows bigger.
This made for an interesting trade-off in physical strength vs. intellectual capacity. It also makes it dangerous for him to grow very big, as he is always at risk of losing control as an effect of his fading intellect.