It depends on your riding position. If the handlebar is about the same height as the seat, a B17 'standard' should be about right. If the handlebar is lower, get a narrower saddle (Brooks Professional is a little narrower than the Standard, and the Narrow is a little narrower than the Pro). If the handlebar is higher than the saddle, get a wider saddle. The wide model Brooks saddle is the B72 but I hear they are getting hard to find, possibly discontinued.

Some people recommend measuring your sit bones. I don't. But if you do, make sure you take the measurement in the position you sit on the bike. Sit bone width depends on the angle of the pelvis.