To get a Western seat size, subract 1 1/2" to 2" from your English seat size. I ride in a 17 1/2" - 18" English, and would take about a 16" Western.
Western and English trees are drastically different. Here is some information about trees:
Western saddles can be very hard to fit because they can't be adjusted. Any fit problems are often well-hidden under the skirting, so even if you're decently informed about fitting English saddles it's still hard to tell about a Western saddle. (Incidentally, my book has both English and Western saddle information--see my site for more details: http://lorienstable.com/book/
If you can find someone who can measure your horse for size, before you buy a saddle, that might save you huge amounts of time and hassle--and likely money too. Incidentally, the company who made that saddle on ebay (http://www.doublejsaddlery.com/
) looks like they might be a pretty good one. But I have yet to see a TB who would take full QH bars!