Most of the dresses at the shop I went to came in the largest size available, then when you picked the one you wanted to rent, they would alter it to your size. I did have some anxiety about how well it would be altered, though. Typically, if you buy a dress, they order it in your size and alter it to your body if needed. Mine was going to be altered from a size very different than what I wore.
Also, I would just like to say that every single one of my friends that have gotten married have bought their wedding dresses and been happy with their decision. I just wanted to share my experience with renting for anyone who is considering it. I also hope you aren’t ashamed – like I was – to tell people that you rented it if you decide to do so. Do whatever is best and most important for you!