I found this wiki page today and thought it was interesting, so here it is:
http://en.wikipedia.org/wiki/US_standard_clothing_size
I think the sizes in this page used to be the standard one back then (at least it was correct for my size of 10 years ago.)
So yah, since I heard a lot of people complaining about not knowing their real sizes (for many reasons) etc...maybe you could check with it?
Hope you have fun
p/s: this page is also very interesting: http://en.wikipedia.org/wiki/Size_zero