A 6-6 guy can probably guard a 6-8 guy without too much problem. But when trying to guard a 6-10 or 6-11 guy, that could be a different issue.
I'm not trying to say that Royce can't function well at the PF, but he is overcoming some size disadvantages. I don't think there's any point in trying to imagine that they don't exist.
Royce is at best average sized for the 3 (average perhaps only in weight, but short for an average 3). He's basically Dante Exum or Alec Burks with a bit more heft and a fair more amount of inside toughness. But he's nowhere near as heavy and strong as a Crowder or a PJ Tucker.
If he succeeds at the 4, nonetheless, great. But he's not what we normally think of as sufficient size for the 4. I think it will be best if, like the rest of the Jazz's PFs, he spends only some time at the position so as not to demand more of his body than is fair.