A horse being "too big" is more of a confidence thing ime than anything else. A bigger horse can be a lot to keep together - especially in dressage if you're a small rider because you can't get your leg around them to lift them - but you learn to work with what you have, if you have the desire to do it.
Mounting is also uh... interesting, if you're a small rider with a big horse. I physically cannot mount my mare from the ground because she's 17h and I'm 5'3 and not very flexible. So, what I did was, I taught her to stand where I put her, no matter where that is. I can mount off a log, a stump (my main mounting block is actually a tall stump), a gate, the round pen fence, the fender of a horse trailer, literally anything I can stand my horse next to. I can stick her in a ditch and if the ditch is deep enough for me to reach the stirrup I can mount from the side of the ditch. If I have someone with me, I get them to leg me up. There are workarounds for mounting a tall horse while being short.
I'm not going to lie though, when my giant plays up it's much more intimidating than when my mum's 14.3 pony plays up...
MAKORA THOROUGHBRED SPORTHORSES