Saddles are probably the most personal item on a bike. You can get measured for the width(and there are plenty of ways to do that yourself if you like; but most bike stores can do that for you). The big trick is finding a shape and firmness that works for you. Some people like quite swoopy saddles, others (like myself) like very flat ones. Almost all saddles have some form of relief cut out/depression to relieve pressure on the tender bits.
A good bike shop will have "loaner" saddles that you can try (or a good return policy) to help you find the right saddle for you. I can rule a saddle out in 5 minutes of riding, but it normally takes me a good 40 or 50km long ride to decide that a saddle is right for me.
If you're seriously saddle shopping,use the resources at your local bike store and then buy from them. This is a service well worth paying for.