That actually sounds about right. I'm 5'6" and have a 19" ride. That converts to about 48 CM, but I have the seat up kind of high. When I stand over the top tube, I'm just about hitting it with my crotch. That's how the LBS guys figured I wanted the 19" bike.
I'm 210 pounds and don't feel that the bike is too small for me. What kind of bike is is? Is it a MTB or roadster that you can ride sitting in a more upright posture, or is it a roadie that you hunch over. Changing your riding posture might make the difference.