Honestly, there are more clues to what he looked in the historical Julius Caesar than there is in Shakespeare's play. What we can tell is that he had receding hair, was relatively thin, and had a prominent nose. He would also have been cleanly shaven, most likely because that was the style for Romans during his period.
Shakespeare does not provide too many clues as to how he wanted Julius Caesar portrayed physically. He does emphasize his physical ailments though, and talks about his partial deafness and his tendency to suffer from seizure but other than that...¯\_(ツ)_/¯.
