It depends on the brands for me...
My current English saddle is a 17(Thornhill), but I just put a new saddle on layaway and it's an 18(Toulouse).
As for western, I don't own one yet, but I've put one on layaway as well...It's a Tucker and I want to say a 17". I sat in a couple others that ranged from 16-17.
My butt obviously doesn't want to pick a shape and stay there LOL
I ride mostly my Billy Cook which is a 16". My Circle Y is also 16".
I do have a Collegiate Dressage that is 17" and a Crosby that I believe is a 16.5". I haven't ridden in either of these saddles in years.