Hmm...
You'd really do well to tag this as an AU. Not because of the Harry-in-Azkaban trope, but because of the business about Harry's size and everyone thinking of him as a baby.
Harry is supposed to be on the small side, sure, but there's nothing in canon to suggest that he's more than a couple of inches below average, and certainly nothing to suggest that at fourteen years of age he was smaller than all the eleven-year-olds. Nor is there any suggestion that Ron thinks of Harry as his _little_ brother.