The most important measurement to me is the top tube length, followed closely by the head tube. For compact frames, you should size by the effective or horizontal (or theoretical) top tube distance. Although I prefer the look of traditional frames, I like the generally longer head tubes on compact frames -- and they make a lot of sense with threadless stems.

In a traditional frame, I generally ride a size 56-57 top tube, depending on the seat-tube angle. I own two compact frames and fortunately both of them are sized in cms. From my experience, the compact frames that are sized S-M-L-XL never fit me quite right. They are always a little too long or short across the top.