Add Belize to Jamaica and Guyana and to the Leeward/Windward Islands and that's about what I would expect under this scenario. These areas have sufficient population to be provinces here, as in post-WWI era each of these three will have more people than Saskatchewan or Alberta did at the time.
For the full wank we'll lump Bermuda with the Bahamas.
But Belize and Jamaica are sooooo different that leads us back to OTL problem pointed out by Dathi. I see your point about the low population of Belize even though it's greater than Prince Edward Island's. How many "Prince Edward Islands" would Canada allow itself to have as provinces, really?
But if we're talking about a post-ww1 transfer I think the answer is easy: none are provinces, all are territories for the time being.