Actually, I need a seat that is larger than 18" - my backside is HUGE! I'm 5'2" and way 265 or so, and I am VERY pear shaped! I had a 20" saddle that my backside loved, but it had an 8" gullet and did not fit my girl at all. There is a really bad pic of her as my avatar. She has a short, broad back and is high withered to boot. Thought about a saddle with Arabian bars, or even a gaited horse saddle - she does single-foot, but I don't know that it means she has to have a gaited horse saddle.
I saw a 20" saddle on Ebay that had a 7-1/2" gullet. It was a cheaply made saddle, but I thought if it fit Dancer well enough to at least do no harm (I could buy a wither builder pad to use between the saddle and a regular pad) I might go with the cheap saddle until I can lose enough weight to fit in an 18" saddle. I do have a 17" saddle, but it is murderously uncomfortable for me - I keep catching myself leaning forward in it because I can't keep centered in the saddle what needs to be centered. It also tends to pinch Dancer's withers a bit. She's game enough to go for a while in that saddle, but eventually it just gets too uncomfortable for her.