A good general rule of thumb, is you want the length of the tank to be at least half the length of the snake. Bigger/longer never hurts, but smaller wouldn't be advisable. Also, corns are a terrestrial species, so even if yours likes to climb you still want more horizontal ground space than vertical height. This will help with proper temperature gradients and the snakes ability to properly thermo-regulate himself. As others have mentioned, just make sure you have hide options for him, and if you put him in a considerably larger home, give him lots of cover and clutter to help feel secure.
Based on what you've said about the tank (it's approximate gallon size and the fact that it's a reptile designed cage) it should be good enough to house your corn for the rest of it's life. So I'd say go ahead and grab it, as long as it's a "long" reptile cage and not a "tall" reptile cage.