Corn snakes are not demanding on tank size. Most corn snakes can be kept in just about any tank from 20 gal. and up. Of course, you have to take into account the height of the tank. For example there is a 20 gallon long and an 20 gallon tall. Both hold 20 gallons, but the "Foot Print" of the tall tank is smaller than that of the long tank. The long tank may be of sufficient size, but it may not be Tall enough, as corn snake like to climb.
So the best thing is to try to compromise. Get a tank tall enough and with a large enough foot print.
Keep in mind, the larger the tank, the more "Clutter and Hiding Places" you'll have to provide. A smaller well covered tank would actually be more comfortable to the snake than a large Wide open one.
I have a 30 gallon tank currently, and my snake is about 4 feet long. He is still growing, and although he could probably stay in the current tank, I will probably upgrade to a slightly larger one in the near future.
