The following is a general guide. This is not definitive but a general recomendation for western saddles. If you disagree, then go with what works for you. English seat would add 3" to the western size. Aussie seat would add 2" to western size. Once again - these are general guidelines.
Women: Saddle Seat Size (Western) 15" Weight 105 to 150 Pant Size 6 to 10
Saddle Seat Size (Western) 16" Weight 150 to 170 Pant Size 10 to 14
Saddle Seat Size (Western) 17" Weight 170 to 210 Pant Size 16 to 20