What size hunt coat would I wear?

I would go to the tack shop and find a coat that fits, and buy it there. I ride dressage, and the nearest tack shop that sells dressage apparel is over 30 minutes away (I've yet to time the drive)! That way you can try on a bunch of different brands and find one that fits (some brands fit differently than others). It always sucks when you find the "perfect" item online, but you find that it doesn't fit.