The girth has to be atleast 2 inches off the ground
Thank you very much Annelies and Emily !!
This is a pretty simple way to size an english girth: https://www.horse.com/content/tips/measuring-an-english-girth-in-five-easy-steps/
The type depends on your preference, and your horse's individual needs. There are leather, string, anatomical, etc. so depending on what you need out of a girth, what you choose will differ. I don't have a ton of knowledge about girths personally, so I would talk to your trainer, barn owner, or someone knowledgeable at your local tack store! You can also do tons of research online into the benefits of different types of girths.
I'm currently riding in a professionals choice ventech girth, and I really like it. It seems to keep my horse comfortable, and it seems to be holding up really well. What I do like about it is that it has elastic on both sides. When you have elastic on only one side, it can create different pressure on both sides of the horse's barrel, and can create some discomfort. The two elastic sides ensure that the girth moves comfortably with the horse.
There are two main categories of straps: long straps and short straps. The choice will therefore depend on the length of the straps of your saddle. Most obstacle stools and mixed stools have short straps, and in this case you will need a long strap. An exception: Mixed saddles with predominant dressage. Dressage saddles with elongated saddle quarters and long straps, you will need a short strap (also called dressing strap).
There are a lot of different girths: standard saddle girth, anatomically shaped girth, special-purpose girth (stud guard), nylon girth, neoprene girth, leather girth and sheepskin girth.
Have a good day,