Size is dependent on your thigh length I think - so go to the saddle shop and try saddles. Also English saddles are harder to fit, some will need restuffing and sometimes certain brands just won't suit you and your horse.
Saddle pads are the least of your worries, some people don't even use them but generally you use a pad to suit the flap cut of your saddle, so a dressage saddle would have a straight front where as a jump or AP would be curved.
Okie I'm just going to tell yall anyways XD so when I bought this saddle (for whopping $50)the lady called it a paso saddle and what she said people would use to do was ride pasos for days sometimes weeks almost nonstop. Eventually the horse would loose weight and the slit in the saddle is for the backbone. Sad storey I know. But also when I bought it, it had like a cover that went over the seat but I took it off because it was UGLY! Hahaha...
That is very interesting Larra! That is kinda sad for the horses. I remember something I saw on tv once. People who traveled long distances put raw meat under their saddles so the pounding action of the horse would break down some of the meat and tenderize it LOL