I got the 2xl based on my waist size (34), but ended up exchanging it for the xl.  I haven't received it yet, so I'm not sure if it will be a better fit, but the 2xl was almost loose in the waist all
cinched up, yet my rib cage needed a lot of loosening in the ties.  To top it off, my breasts were nowhere near enough to fill up the cups.  I'm a 36B and I'd guess you'd need to be a sizable C to make it work.  I'd recommend going off bust size if you're smaller and don't mind having to loosen the back a bit.

Update: nope- xl was still too big for my boobs. It might have been better if the straps were adjustable, but as is it was pretty unflattering.