Join Date: Nov 2011
Location: Beautiful Pacific Northwest
• Horses: 0
Do a search for English saddle seat size calculator, and you should find several different tables, calculators, and measurement ways to estimate your seat size. Do several of them and see what result you get the most. That could be your starting point.
For English saddles, you're looking for the seat to fit you well, but the flaps also need to fit your leg-length, so there are a lot of variables. Also, the saddle design will make you fit various saddles differently. I "normally" ride in a 17.5" saddle. However, my favorite, and old, flat Passier, fit right in a 17", because it's actually measures a 17.25", and the flat seat makes it roomier than a deep seat. In a deep seat, I sometimes like a 18" better, especially if the flaps are on the shorter side and I need to accommodate my freakish legs.
You don't mention anything about your build. At your age and height, and the current size addle you use, I would guess that you'd probably be looking at a 16.5" saddle, give or take 1/2" depending on the saddle design.
Make sure that anything you buy has a return policy, because even if you determine your ideal" size, you might not like the fit when it shows up on your doorstep.