Join Date: May 2009
Location: Winnipeg, Manitoba, Canada
Really, it's the exact same as you'd do with driving, just with different gear. As a trainer of racehorses, I'm sure you know exactly how to condition them to a harness and cart so the theory and principal are going to be the same for a saddle and weight.
As for size, most adult horses in most breeds are capable of carrying fairly extreme weight. Heck, even ponies are, actually they're built to carry even more weight but in a relation to their size. Icelandic horses are technically ponies and are able to carry full grown men with ease - it's funny to watch, I swear their feet almost touch the ground sometimes!
Unless you're pushing over 300lbs, a Standardbred should have no problems carrying you. Think about it - Quarter Horses aren't big horses, most rodeo type bred ones only push between 15hh and 16hh and they're regularly ridden by 6'0"+ men that have to weigh EASILY 250lbs in most cases (especially those bull doggers!)
My friend actually roped off a Standardbred once! Favorite roping horse he ever owned, and everyone always laughed at him until he kicked their butts! He was a solid 6'0" tall and over 200lbs, and had no problems!
That's a darn solid mare you have there to, especially if she's only 2 right now. I wouldn't worry at all about size!
I hope God tells her to smash her computer with a sledgehammer.