Seems like a tricky situation.
The best advice would be to try it on. If it is possible to find a day to drive out to the tack shop, I recommend it. Breeches vary in size and different brands fit differently.
Is there a horse consignment store near you maybe?
That would be useful too, and it would have some good prices.
If you really can't drive out to the tack store, I recommend reading in the Dover Saddlery catalog, I believe they have a sizing chart in it.
Be confident. Too many days are wasted comparing ourselves to others and wishing to be something we aren’t. Everybody has their own strengths and weaknesses, and it is only when you accept everything you are — and aren’t — that you will truly succeed