Join Date: Dec 2011
Location: Ontario, Canada
I wear a size 16 jean, (Canadian sizing, not sure how it differs from any other countries), and I'm currently in the process of teaching my 16.2 TB gelding to jump; and other than being incredibly klutzy, he's been doing well - with no issues!
Honestly from all the horses I've ridden (used to work at a schooling barn, so I've been on a few), I've never had an issue. It's all about building up the horse's stamina, just as much as it is about building up your own.
You should be just fine :)