Many men weigh up to 300 lbs and find horses to efficiently carry them.
Some of the internationally recognized experts on equine anatomy, conformation, biomechanics, and exercise physiology have published information and advice on this issue. The antiquated idea that an arbitrary percentage of a horse's own body weight can be accurately applied to calculate the weight-carrying capacity turns out to be false.
In fact, any horse weighing over 1450 lbs is now known to be poorly designed to carry any more than its own enormous body mass since the amount of bone/tendon circumference required to support any additional weight on the horse's back has never been attained by any horse of any breed. In other words, massive draft breeds weighing over 1450 pounds are not ideally suited for carrying any weight on their backs without risk of soundness issues, let alone being suitable to carry heavier riders.
These horses are designed to pull heavy loads, but are not well designed for carrying weight on their backs for any kind of athletic performance. It is also harder for these horses to dissipate the excessive amount of heat yielded from the massive muscle metabolism required during their workouts.
For optimum weight carrying capacity, the way a horse is put together is more important than anything else. Of course the horse should be skeletally mature enough to carry weight.
Ideally, the horse should measure at least 8 inches of bone/tendon circumference just beneath the knee (carpus) for every 1000 pounds of his own body weight.
If an ideally fit and nourished adult horse weighs 750 lbs, you'd want him to measure a minimum of 6 inches of bone/tendon circumference (one inch for every 125 lbs of body mass) below his knees. On the other hand, if an ideally fit and nourished adult horse weighs 1500 lbs, you'd want his or her bone/tendon circumference to measure at least 12 inches just below the carpus, which realistically isn't ever going to be the case, with only 10 inches of bone/tendon circumference on a 2000 lb draft being the average maximum.
Taller is not better either when it comes to ability to carry weight. According to equine anatomist and conformation analyst Dr. Deb Bennett, any horse measuring16 hh or more and of low enough body weight to have sufficient bone mass will seldom be broad enough through the loins to efficiently support a rider's weight. A lower center of gravity also assists a horse in balancing the weight on his back, so it is best to find a horse who is just tall enough to accommodate you, and no taller than is necessary to fit with your height and build.
A short, broad, smooth deep loin is ideally suited for carrying weight on the back of a horse. The circumference of the heart girth should be virtually the same as that of the loin/groin for maximum strength. The lumbar span measured from the last rib to point of croup should ideally be no more than than the width of your hand, and the total length of the back should be proportional to the fore and hind quarters.
Other qualities to look for according to the experts include correct angles of bones below their joints, and correct bone lengths. Short bones come with shorter tendons and ligaments, and longer bones come with longer tendons and ligaments, and when the joint angles or lengths of the bones are not correctly balanced for optimum function, all of the musculoskeletal structures are more vulnerable to injuries.
Some other qualities to look for according to Dr. Bennett include:
"....A neck set high on the shoulder, with a shallow vertebral curve at the base of the neck,
moderately high withers, with a peak that lies well behind the horse's elbows, (and)
a pelvis that constitutes at least 30% of the body length and slopes from 18-22 degrees....."
The problem you may encounter is the fact that many people out there still blindly follow the antiquated 20% rule despite the fact that it is patently flawed.
A taller, heavier rider should try to find a horse under 16 hh that weighs less than 1450 lbs and is built the way I've described. Hopefully you will find a rescue facility with knowledgeable personnel who stay utd on what equine research has taught us over the years.