Ridley's have real high bottom brackets. I ride a 58cm road bike and tried a 58cm Ridley and had almost no crotch clearance when straddling the bike with both feet on the ground. I bought the 56cm and it fits and rides great, but I could still use more stand over clearance. Fantastic bike though (SuperCross)
Had a Ridley last year. My road bike is a 56 and the cross bike was a 52 with little SO height. If you are buying from a shop ahve them throw some wheels on on the frame and do a SO check. Consider the effective TT length the onyl true measurment. Also, check if you can get the frame onto your shoulder too. That's an important check for a cross frame IMHO.