I kind of know what you mean. For the longest time I rode ponies and only ponies. In all truth, I actually cried when my instructor tried to put me on a horse. (I was like, 6, but still!)
I'm about 5'5'', very leggy like you are, only I weigh a bit more. Haha.
My trainer has a friend that adores me. She has this huugee, horse named Jasper. He has lines about two generations back of pure Belgion draft horse. So yeah, he has his size down. But the thing is, he's very lazy. Been under saddle about a year, and the best way to ride him is bareback.
The first time she asked me to ride him I hardly got him to move. After all, the only person who had ever rode him was his owner, so he didn't exactly trust me. I got in a nice ride and then didn't ride him for a while. Last week she did a sneak attack and told me to go get my helmet, hand her the horse (I was cooling down Clyde), and take the crop. Before this, when I first saw that she was riding him bareback, I went up to her and was like: "You're riding him bareback?!?!" "Yep!"
Now I'm perfectly fine with it. I mean, it's really amazing how much better he actually behaves when he doesn't have his saddle on.
--My Point Is--
It just takes some time to get used to. Don't push yourself so hard that you feel really really nervous, but push yourself enough that it's not what you're used to. Take a chance. ^_^
Learn to sit back a bit more, put more weight in the pockets of your pants, and just enjoy it! Personally, I really like their large movements and gates. I can go to my old Dressage pony to Jasper in a flash and still know what diagonal I'm on and am very used to the way they both move. Give it some time.
Also, can you just ride a larger horse for a while before you actually buy one? That way you can get used to it before you spend your money on a horse that you may not even end up loving?
(I have no idea about the strides thing. I'm a Dressage freak, haha. Sorry!)
And for the reference: