Here's my biggest pneumatic one:
http://members.toast.net/rikoster/photos//Photo0026.jpg
About 7 feet long, 4" chamber and 3" barrel, should be able to handle about 250 psi but only tested to 100 so far. Ammunition of choice is actually a 16 oz beer can filled with water and frozen, stay the **** out of the way when it goes off though. It'll do a hell of a thing to a Nissan Sentra at 50 yards. Another fun thing, stuff a Nerf football down the barrel and catch your buddy in the back with it.
It takes 2 people to efficiently aim and fire it because of the size, i almost broke my collar bone once by charging it with air, filling the barrel halfway with water, and setting it loose. Kicked so hard it slipped out of my hands and the brace at the end (not in picture) caught me on the shoulder and knocked me down. Won't do that again...
Also have a small one with a 1" barrel that will shoot an after-dinner mint through a coffee can, load about 15 of them down the barrel and they break into pieces as they are fired, that's some squirrel killing shrapnel right there.
Ever shot a golf ball through a car door? Got one of those too, damn thing was a poor design though, about 14 feet long. Golf balls are shitty ammo though, they bounce way too much, catch something solid and it'll come back at you at about half the speed it left the barrel, it'll leave a mark.
Everything I have now is pneumatic, I had built a 2" barrel cannon powered by propane and a MSD ignition with 2 spark plugs, still not as reliable as air and a hell of a lot more dangerous. It's just hard to keep them consistent shot after shot.