There are lots of ways to measure them yourself. I have to run catch my bus, or I'd search down the link. (there were a couple threads about measuring)

I do the "sit on a piece of paper on your chair, put your finger at the outside edge of each bone, and mark the paper there with your fingernail" to get outside to outside measurements.

The rule of thumb is to add 1 cm or 2 cm to each end of the outside measurements to give you a ball park of saddle width to try. (mind you, this is a ball park estimate, it's not as important as how the saddle FEELS!)