When you say “full length” do you mean a bag that fills the entire main triangle of the frame?
I’ve a Wildcat Ocelot which is the half-frame bag. This fits both my hardtail and my CX/commuter which while not planned is handy. I’ve the XL which is pretty big (4 litres I think) and can get a lot of kit in there. My wife also has an Ocelot but the small and there’s much less room – I made up some sections of pole for the tarp from old tent poles, they fit my bag but not my wife’s 🙄
I’ve also got a Wildcat full frame bag on the fat bike. I bought the bike and bag from Ian Barrington whose Beth’s husband. The bag is obviously made to fit that frame which is why he sold them together. I think it’s the prototype for their fat bike frame bag. As well as being full frame it’s significantly wider so can hold a lot more kit.
Generally half-frame bags are stock – measure your frame and chose the appropriate size. Full frame bags from the likes of Alpkit and Wildcat require you to make a template of your frame showing bottle cage bosses, etc. and the bag is custom made for you. If you’ve a Stooge then Wildcat at least know the template. Not sure how the “standard” full frame bags cope with all the different angles and frame layouts.
Finally, get some frame protection tape. As soon as any grit gets in behind the straps it will strip the paintwork.