Anything under 17" is considered a youth saddle anyway, so it's time to move up into a 'big girl' saddle.
I had a 19" Stubben. I bought it off CL, because the owner stated it was an 18". That was the
biggest English saddle I have ever seen. It was obviously custom made for a man, because I measured the seat and it was a true 19".
Trust me, you'd absolutely swim
in a 19" seat. 17-17.5" is your best bet. I ride in a 17.5" seat, and although I could do fine in an 18" as well, a 19" is much too big for the average woman.
I sold the 19" on to someone else. Don't know if she kept it or not, but she was thrilled to get it because it was German made. If it had been smaller, I'd have never sold it.