I ride a 51cm WSD on roadbike and a 49cm men's CX bike. Curiously, the top tubes are the same, which I think is the key to getting it to fit. Test ride everything you can!

That being said, if you're not riding the bike much more than the race duration (usually an hour or less), then you don't have to be as precise about the fit as with a road bike.

-- gnat!