My dd is a size 1 too. I recently ordered 2 pairs for her to choose from - one pair was really padded and the other was more like converse, so no padding at all. Although both were size 1, they were massively different in that the unpadded ones came up much bigger, so I would say to size up if you get the getthe padded ones ( which are nice and supportive) and get size 1 if not. We kept the padded ones and returned the others as they were enormous on dd.
I would also recommend getting the ones with 2 wheels, esp if your dd is new to heelys.