Thanks.
Bigger is better with gars (besides babies, of course), so I would go as big as you can. Try to get a decent-sized footprint as well as a decent depth in order to keep the pond at a more stable temperature as well as provide a larger overall volume of water.
Kevin, would you agree that 12' x 6' x 4' (LxWxD) should work pretty well for a few average captive alligator gars? I would double-check on the depth, though, as you may need to go deeper in order to compensate for the colder winters.
Also, keep in mind that you're looking at sinking a decent chunk of money into this project, so be sure that you can swing it before starting; I doubt that your parents will be very pleased if you have to stop the project part of the way through and leave the yard torn up.
Ryan I wopuld agree that a 12' x 6' footprint would work pretty well for a group of captive raised gators. As far as the depth i personally wouldn't risk overwintering outside, unless going quite a bit bigger in pond size.