First, do you want Large or Small caliber: .45/10mm (Lrg) or 9x19/.40S&W/.357SIG/.45GAP (Sml)
Then you need to choose pistol size: Extended, Full Size, Compact or Ultra Compact.
Slides are interchangeable within their own particular group/category if you can follow what I'm saying. Hope that's clear.
There's lots on the net to do with this - Check Glockmeister.com