Mine is about 13" long around the ouside of the curve(about 10 1/2" internally, measuring the centerline) and is about 2 1/2" in diameter at the base, holds about 3/4 of a pound I believe.
Remember when buying a horn that you will end up trimming the tip and ends, perhaps quite a bit, to get about an inch of solid horn to drill the spout in and to get rid of any flaws on the butt end. I'd probably go for a horn over 18" if possible.