it really depends on a few things such as the length of your leg (for example, i had to buy a special saddle because my thighs are too long and my calves are too short!), and also the size of your seat !
The general rule is if you sit in the saddle and you can fit 4 fingers behind your rump and still feel the saddle, your good. Go to a tack store that sells saddles and they'll be more than happy to size you into a saddle!