As Pedallleur says, the important thing is to get a saddle of the right width. This means it matches the width of your "sit bones" (aka ischial tuberosities or protuberances). Our pelvises are generally wider than men's, for obvious reasons, and so we need saddles specifically designed for women (D4W, WSD, etc). It makes no difference whether you are fat or thin - the bones will still be in the same place!
Women's saddles generally come in three sizes:
Small (approx 130mm)
Medium (approx 143mm)
Large (approx 155mm)
Small and medium tend to be better for road cycling, where you are bent forward.
Medium and large are preferred for MTB, with a more upright position.
So the first step is getting the right width. How do you decide? Well, as HTFU implies, the lads in your local bike shop will be delighted to assist
. But check first that they actually have a buttometer (much nicer word than arse-ometer). There's more information on the Specialized website.
Then you can start to look for a suitable saddle for your specific purpose. But nothing beats actually trying out a few different saddles, though!