I am 6'8" and pretty close to 300 myself and I find that if I condition my horses and I don't try to ride one hard day after day my size has little effect on them. It also has alot to do with how well you ride. If you flop around like a bag of potatoes you will wear out and sore your horse faster. If you ride a quality saddle with quality pads and you ride well then you should be fine. Pay attention to your horse and let the horse tell you when it is struggleing.
There's nothing like the Rockies in the springtime... Nothing like the freedom in the air... And there ain't nothing better than draggin calves to the fire and there's nothing like the smell of burning hair. -Brenn Hill