You definately want something higher than 30". If you ahve a female cham you can get away with a 3.5' cage, but that is really pushing it. I would go 4'h for a female, or 5' for a male. as far as length and width, 2x2' is good, for a female you can get away with 1.5x2' but again that's cramped. For a male you should go 2x2.5 or 3x2. chams aren't THAT big, but they use up all of their cage if it is properly furnished. mine is 2x2xalmost 4' high, and it is pretty cramped, i have 2 trees that won't fit in the cage that my cham climbs on when i'm home
sorry this is long. I'm tired.. hope it makes sense 