It depends on their back length not really how tall they are. I had a beautifull saddle that was just to long for my arabian. I had to get a really tiny saddle for him. I bought mine from someone at the barn after trying it on him though so I'm not much help!
Well the saddle seat depends on the size of the rider. Example I am fairly small and I sit a 15" seat. Now depending on the length of your horses back depends on the saddle. If you are going to be riding western, which is my seat, you can choose a couple different style saddles. The have the barrel saddle which is a safe seat with normally a rounded back, which looks nice on a shorter backed horse. But if your gelding is long you may look more for a western pleasure or roping saddle with the square back. They are a bit longer and don't look lost on a long backed horse. I hope that helps some. Have fun. Horses have been a huge part of most of my life. I cannot imagine not having any.
There are three things that you should consider when purchasing a western saddle.
1 Type of horse (1/4 horse, arabian, tenn. walker, etc)
Different saddles are designed for different horse body tpyes. Does your horse have a round barrel shape like a roping 1/4 horse or high withers like a tenn. walker.
2 What activity are you using the saddle for ( general use, sporting events, etc.) If you are going to be roping off of the horse you want a roping saddle, if you are cutting then you want a cutting saddle, barrel racing then a barrel saddle, or just a general purpose ranch saddle.
3 How large or small are you. You can get seats any where from 13" to 18" depending on your body. What you feel most comfortable sitting on. A 225 lb. man would not fit into a 13" or 14" saddle and would not a 75 lb. girl would slide all over an 18" saddle. Also if you are a teenager you may want to consider the synthetic saddles, they are lighter, they hold up a little better in the rain and you don't have to oil them.
Think of what you need and try out some saddles from other people in the barn. Ask the person you are purchasing the horse from what kind of saddle they used on it and then adjust the seat to your body.