I agree with NRHA reiner. I'm 5'7" and 140-145 lbs... I ride in a 15 1/2 inch saddle.
Size of the saddle DOES matter. A too tight saddle will throw a rider forward, shift their legs back, and cause them to lose balance.
Be careful with riding a youngster. Larger colts tend to mature a little slower. So, don't let his size justify his riding program. Trotting is one of the toughest gates on a horse with a heavier rider. I'm not trying to bash you here, I've just seen colts go down with over weight riders. I would definitely focus on getting healthier and really working hard so that you can enjoy your horse. I would also try out a bigger saddle and see if that holds you better. I bad fitting saddle, for either horse or rider, isnt worth having in the barn. I know I can 'settle' for a 15 inch, but I personally prefer the 15 1/2.