I don't have a small frame, but I'm 5'6 like you -- big rib cage though.
Best idea I got from that website by the way was to find a picture from Playboy with someone whose boobs you like AND their frame seems to sbe similar to yours. We got a subscritipon online for a month and found a great photo and I'm really happy with the information my surgeon gave me. I took the picture to him and was like "I think she looks like me... that's the proportion i want to have." It was a cool experience because then I really had a flimpse of what I'd look like.
And don't worrya bout going too big -- I was scared to get mine and when I saw them the first time, I knew they'd be real modest.
Now I don't think they're real big at all (I'm a 36C, 425ccs were put in) so it's surprising how you feel about wanting to hide them