I want to use a rope girth or a Leather felt lined one.
I want to buy everything before I get my horse but i'm told I need to wait until I get my horse.
What's the average size girth/cinch?
Can I get one before getting my horse?
It would be best to have your horse first. I read an article awhile ago that recommended putting the saddle on the horse and taking a tape measure from the rigging on one side, under the belly to the other side. Take 16 from that measurement and the remaining # is the size girth you need. I have Arab crosses and I use a 32" on them most the time.
You should hold off on buying your tack until you have the horse first. I bought a show saddle for my show season years ago for the school horse I was going to buy, the sale fell threw and I never bought the horse. He took a 46" cinch. By a very lucky chance that saddle fit the horse I eventually bought years later, however the cinch was 10" too big and caused both my horse and I a lot of problems, such as saddle slipping which lead to discomfort and lots of bucking/rearing. It's better to get your horse first and measure him properly so that you can buy him proper fitting tack. I know buying stuff for your first horse is exciting, believe me I still go hog wild at the Pleasant Ridge tent sales, but it's better for you and your horse to have tack that fits and is comfortable.