Sandra,
sorry for not getting back to you eariler, my laptop kept overheating and shutting down after about 10 minutes of use! I kept trying to post replies and I couldn't get that far.

I am going to make a huge guess that the 50 is going to be to big for you, and that you are going to want a 48.
I am 5'6 and I ride a 48, technically I have been told I should be on a 50- but it leads to me being to streched out(my legs are long enough about a '32 insteam, but the torso is not).
Ride the 50,see how you like it- if you love the bike but feel a little stretched out, the LBS should be able to get one in, in your size, and built up within a few days.