Wow! How exciting that you are getting your first horse.....I can completely relate to were you are coming from, seeming as I’m 14 and 5”4. I don’t own a horse (yet) but in my lesson times I ride a 14.2 22 year old QH mare, and she gives me quite the challenge occasionally, but what mare doesn’t? I know in my horse shopping experience, lots and I mean LOTS of more experienced riders that me say “you don’t need a 16.2 Horse” “you’ll die that’s so dangerous” especially my experienced horse friends, so I don’t talk about height with them, and they know I want a warmblood and they describe a 16.2 warmblood as a 17 Thoroughbred.....I have no idea!!!!
But in reality, everything comes back to temperament. You will find 17.2 thoroughbreds that you could put a baby on, you will find 13h snobby English riding ponies that are girthy as hell and are kickers/biters. It’s what height you feel comfortable with. Take this for an example, friend of my does Advanced dressage, she’s 5”3 and rides a 17 Hanoverian Mare, she lengthens her stirrups so she can give sufficient aids to the horse. If you feel like you can handle a 18 hand bombproof warmblood then buy all means, buy it! But if you feel like you want a 14.2 frisky Arab then go for it!! Personally, I don’t like riding horsss that go over 16.3, because they give off this scary nature.....and that’s my personal opinion.