Update: After a bit more research it appears Mighty Man didn't actually get his shrinking/growing powers until issue 12 of Amazing-Man comics (May 1940), before this he was just a giant. This is after the first appearance of Dollman as mentioned in another answer, but I'll leave the answer up, as I think it still adds to the conversation.
Maybe Mighty Man who first appeared in issue 5 of Centaur Publishing's Amazing-Man Comics dated September 1939.
Mighty Man was more than twice as tall as most people, and super-strong even for his size. Explorers of the American Southwest found him in the so-called "Valley of the Giants" (no relation), which had apparently been settled in the previous century, tho the settlers had all died, leaving the young giant the only survivor. Back in civilization, the future Mighty Man submitted to an experiment conducted by a Dr. Robert Hilldale, which brought him down to normal size, but also gave him the ability to shrink and grow at will.