I think I figured out what the confusion was for me. The small (which is a size 51cm) has a 53cm head tube. The website I ordered from just listed the head tube size, not the actual size of the frame, and so I thought the "small" was a 53cm and that the "extra small", which has a 51cm head tube, was a 51cm frame. I did test an actual size small at a LBS and it fit well. I'm 5"5 or so, with an inseam of 28 or so inches.

Can't wait to get it!!