I am looking at a 99 GT Avalanche and I know I have to ride to get a feel for it, but before making an appointment for a ride I wanted to check wrenchscience.com They are saying a 15"-16" for my build:
You're 6' tall, a 15"-16" frame is way too small. Riding a bike that is too big or too small can be bad for your knees. Try at 17 or a 17.5.
Being a fellow 6' tall person, unless that frame is DJ geometry which is designed to be small, which this gt isn't, it's a xc bike. For example, my kona is a medium and it is a 15.5 but measures out to be a 17.5, My Rocky Mountain Vertex is a 17.5. Wrenchscience has screwed up measurements before, best way to get an accurate measurement is to go to a bikeshop to have them fit you for a bike in person.
You should have the saddle set to the point where it is level with the stem at least. You'll likely need 175mm crankarms for the bike. For stem length anywhere from 80mm to 100mm. With the stem it is personal preferance.