My personal guideline (based off of 2 years worth of saddle-swapping and 14 saddles) is when you're looking at modern synthetic saddles, you want one that's at least as wide as the center to center measurement of your seatbones.
If you're looking at a Brooks, go with one that's a little wider than the measurement from outside to outside. That way you won't make seatbone contact with the frame of the saddle.