In the first Oz book, The Wonderful Wizard of Oz, the Wizard is revealed to be a little man, barely taller than Dorothy, who is as bald as an egg. In every book thereafter the text describes him as a little man who is as bald as an egg. The illustrations by John R. Neill, however, show him with a lively fringe of hair. He’s also quite a bit taller than in the original Denslow illustrations.
As you can see I prefer a version with the lively fringe. If asked to explain the difference I’d say that the Wizard shaved his head during his first sojourn in Oz. A shaved head would have made it easier for him to slip on the disguises he needed to maintain his air of mystery.