Team Chasing and Showjumping require fitness and, personally I'd look for something larger than a welsh c cob type.
It'll depend on the pony or horse though.
I'd say you would be fine on anything from 14 to 15.2/16hhs. You need to consider the breed/type, health, conformation and fitness, especially if you're going to be doing those sports.
Some of our natives have round barrels and take up a lot of leg, however they can be short in front making you overly large on top, making it hard to balance. The narrower types will have your legs hanging low making it difficult to give aids.
Different sizes can cause discomfort for the rider as well.
Also consider that the saddle that fits your height and length of leg could be too long for a smaller pony's back creating pressure points.
Your weight might be an issue given what is required to team chase and jump even at lower levels. You're not asking it to go for a hack around the lanes at the walk for thirty minutes.
Go too large and you'll find youself over-horsed with similar issues such as balance and control, perhaps even frightening youself.
It's something you'll need to work out for yourself with support from your local school/instructor. Try as many ponies and horses as possible and see what you're comfortable riding and controlling.