Unfortunately, and although it's not the answer you're looking for... it's true, it's all about proportions, and we can't tell you what size you'd be.
I, for example, have no butt. I could gain like.. fifty pounds and it would all be in my hips, thighs, arms and face, and my butt would stay flat and stupid. (Yes, I just called my butt "stupid.")
As a result of that, I sometimes fit into smaller sized pants than someone who.. proportionately, is "thinner" than I am, but happens to have a bit of a butt on 'er. Believe me, it's not as ideal a situation as you would think.
However, if you stick to it and start shedding some of those pounds, you'll undoubtedly see a difference! I'm 5'4" and 21, and at about 167 I fit into a size 13/14. So if you're built anything like me, you'll probably be down a good few sizes.