That size tank, which is basically the size of a 10 gallon tank (actually it sounds like it might be a 15 gallon tank, but I'm not too familiar with those), is okay for a hatchling/juvenile cornsnake, but the minimum usually suggested is the equivalent of a 20 gallon long tank, which is 12" deep x 30" wide x 12.5" high.
I am currently housing each of my 2002 snakes in 10 gallon tanks and they are doing fine in them. They are approx. 25 inches long, and I figure that they can stay in them for a while still. I'll probably move them to larger tanks when they reach 30" long or earlier if they start to look obviously cramped.
My 2001 is in a 20 gallon long and so far he is doing okay in it, but it seems like he will be too big for it when he is full grown and he is only a little bigger than 3 feet.
Just remember that floor space is more important than the height of the cage.
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alstiver@hotmail.com
1.0 '01 Hypo snow cornsnake (Tesla)
0.1 '02 Ghost (pastel) cornsnake (Banshee)
1.1 '02 Bloodred cornsnakes (Desi and Luci Too)
0.0.4 '03 American toads
0.0.3 Goldfish (Kabuki, Isamu, and Yuki)
1.0 American Eskimo mutt (Rusty)