jyyanks. In the F5 the 48 is the smallest frame. I'm thinking you're looking at the ZW series since that one is done in petite, small, medium and tall. The 48cm F5 would be similar to the petite in the ZW although the geometry is different. For the F series the 48cm is 700 wheels. Looking on Felt's site they only list 700 wheels in the spec section of the ZW series. It looks like Felt lists the women's bikes at petite, small, ect and the men's bikes in cm.
Here's a link to Felt's website:
http://www.feltbicycles.com/USA.aspx
Hope that helps a bit.