Good on you for getting a binder!
The answer is already there, but I'm going to provide an explanation if it's needed, as nobody's done that yet.
For clarity: when in-between sizes, a bigger binder is better because there are already risks that come with binding due to the constriction a binder has on the body. It's safer to get a larger size, even if it doesn't bind as well, because those risks (broken ribs, in particular) increase if you're getting something that's too tight.
I'm sure you've already heard about that, but you asked about size so there was uncertainy over here; it's better to know why rather than just what, in any case.