I've been recently branching into the Wide World of Western too, and I've learned a few things so far:
your English saddle size minus 2 is your Western saddle size.
- There are a TON of different types of Western Saddles! Equitation, Barrel, Roping, Reining, etc etc. They all fit a little different and you could be a different size in a different type. I sat in about 6 different 15" Western saddles at the tack shop and some fit, some didn't. The height of the horn and the saddle's overall weight made a difference too.
- When you sit in the saddle, there should be 2 finger's width between the front of your thigh and the pommel/swell. As opposed to English's 4 finger's width between your butt and edge of cantle.
I ride in 16 and 16.5" Close Contact english saddles and I seemed most comfortable in 14.5 and 15" Western saddles. I got the impression that 16" Western saddles are the most popular/average size from the salesperson I was working with. She also recommended I visit http://www.western-saddle-guide.com/
to learn more!