If you're looking at WSD bikes, a 54cm may be the right size. FYI, I'm 5'7" and ride a 54cm unisex frame. Anyway, some manufacturers basically make their WSD bikes a size down from marked compared to the unisex line, though there is some variation, like seat tubes may be on the longer side. So, if I got the women's version of my bike (a Specialized), I would get a 56cm.

I would guess that 50cm is fairly small for you, and it might mean you'd have some pretty aggressive saddle to bar drop (i.e., you'd need a lot of seatpost, and the head tube will be short). A 54cm could be too big. 52cm may be right on. This will all vary by manufacturer.