2.2 Liter airpots is the standard. Glass lined keeps the coffee hot longer, but they also break easily. The S/S lined airpots are definitely more maintainance-free. The lever style airpots are the best in my opinion. Much easier to use than those original push-button types.
2.2 liter is good. The key to keeping it hot, aside from having it glass lined is to "prime" it. I fill the airpots with hot water before the first brew and let it sit for a minute or 2. This brings the internal temp of the glass up to temp. Empty out water and brew into pot. Now the coffee will be brewinging into a primed 100+ deg. pot and won't be going into a 60 deg. pot (I'm up north by the way ) immediately lowering its temp.
Great idea in pre-heating the airpots first before brewing coffee into them I guess that is the same idea in espresso drinks is shooting the shots of espresso into warm glasses by having the glasses sit on top of the machine to keep the glasses warm. I also heard if you want to keep your coffee hotter in your thermos or travel mug is to pre-heat the item with hot water first .