I'm 5'3" and was those sizes a year and a half ago. Actually I was a 20W in pants, and squeezing into those. And I fit on Forbidden Journey just fine. Was sent automatically to the end seats, but that's OK.
I'm sure I would have fit on the bigger rides, too. I didn't, because I didn't want to go and DS wasn't tall enough yet to go. Oh wait I rode the Mummy and it was fine.
I personally think the sizes you're in now are generally OK for Universal. It's just when you start going above them that things get harder to predict.
don't be afraid of the test seats! the seats are there to help you, and no one is paying attention to you. (and if they are noticing you, then they are scum and not worth you wanting them to like you, anyway)