Sheik, I don't think size is the issue regarding effectiveness as a five eighth, its mobility and play making.. Look at Wally Lewis, probably the best 5/8 in the game, and he was built like a forward and in fact played lock often.
A five eighth's aim is to set up the backline in a manner that will give them space to be effective. Schuster looks to have all the skills needed to be a top tier 5/8 and more. His passing game, of what I've seen is exceptional, he's got power because of his size and the speed of a centre. His defence is strong, he's one of those players who seems to have time and he's deceptive in what he intends to do.
Croker is hardly a small man and we've noticed how small men, unless they have that Johnny Gibbs evasive skill and speed are gradually vanishing from the game. Foran isn't tiny yet even he suffered from being knocked about by bigger players. What you want from a 5/8 more than anything else is to set the attack in motion. Schuster, at least at junior level is a standout in that department and that's why he's been awarded year after year with being one of the best on the rise.