With english saddles it's a combo of your thigh and seat size. But different brands fit different ways. I'm a petite 5"2 with average legs for my height and I use a 16.5 in most brands, or a 17 in most of the french made saddles. I'm not sure about western sizing! If you're looking for a saddle I'd go to a tack shop if possible and sit in as many saddles as you can. Hopefully they'll have a knowledgeable saddler there who can help you.
Dumas' site is one that I also recommend. What I look for in fit is some room at the cantle and a fist of space (up to about 4") between my belt and the swell. If my back side is up against or squished against the cantle and I have little or no room in front, then the saddle does not fit. It may give you the impression of security while sitting there but will be very uncomfortable on a moving horse besides restricting the movement your body needs to keep your horse balanced and you in rhythm with him.
(I think that's me in your avatar - just before my horse came over on me - I can still feel the pain!)
No, actually I saw a picture taken of some woman from the back, sitting in a saddle. Well lets just say it wasn't a flattering picture and It just got me thinkin and thinkin always gets me into trouble. Well the picture wasn't of me though!!
I'm 5'5 and sitting at 120lbs. I normally should take a 17 seat but got that extra inch so a 17.5(better resale value). My guy is going to take (very likely tho not for sure tho) a wide tree which, is what I just ordered and what it was fitted as.