Hey NLB, I'm sure you already got your bike by now. But just wanted to confirm, my girlfriend is a bit under 5'6 and has a 54cm specialized that we need to move a few parts to get it to fit. I felt she is in between 51 and 54, but went with the larger one because we can make it work and the deal was hard to pass up.

So yes, the 51 was the way to go!