I haven't read the other posts, but I just wanted to chime in.
First...you're really, really tall!!! You would need a horse with good height, and a good sized barrel...basically, rib and stomach area...to take up your leg, alone. Otherwise, you might be dragging your toes on the ground. (i do not mean that to be offensive!!!)
I had a friend out who was 6'6 and when I put him on my 17 hand horse, with a large barrel, they balanced each other out. In pictures, they looked like an average sized guy riding an average sized horse. (i will see if I can find a pic of them)
Next...and more helpful to you, I think. I know a man who stands 6'4" and was over 400lbs, when he started riding. He started hunt seat lessons on a Percheron (draft horse) who stands 16.3 hands, has really great bone and a great compact build, and later bought this lesson horse. They've been riding together for longer than the 11 years that i've known him. He has said that the one thing he wished he had done differently was started with some physical activity before starting to ride. He would take half hour lessons, but they were completely exhausting to him. He said the worst part, for the first few years, was his balance, and the horrible pain from not using his muscles (the gentleman has a fairly stationary career). He said within a month of daily walking, yoga and a eating one salad per day, he could feel the little differences in his riding. He slowly built up the amount of walking and yoga he did, and it slowly helped him become a better rider. Today, he walks at least 5 miles per day, does 2 hours of yoga...and started his own horse, goat and sheep farm. He does all his own work on the farm, and balances that with his "regular career".
Based on his experience and all, this is what I would suggest.
Before "jumping in" to riding, start exercising. I would suggest what the gentleman did...daily walking and yoga. I guess the daily salad doesn't hurt, as well...lol. He said he started out with 10 minutes of walking followed by 10 minutes of yoga, and built up from there. He said he hated the daily salad. Remember, the man I got this from started out over 400lbs. He's now around 260lbs.
When you think you have built yourself up enough, find a trainer who you are comfortable with. Start your lessons. If you're lucky like the guy I know, you may even be able to purchase your lesson mount :)
Knowing his success, I talked to him about this all, recently. I have a medical issue that required medication that "blew me up". I am 210lbs, standing at 5'5". I just turned 35, and find it more difficult to drop weight, or even maintain what I have now. The gentleman I speak of is 55 and told me that age does make it harder...so not what I wanted to hear...lol.
I hope sharing this gentleman's experience will help you, on some level!!