I agree look at some caresheets and they should answer your questions.
If your buying a baby one keep him in a smaller tank until he outgrows it. Snakes tend to feel more secure when theres not so much area provided. A hatchling or juvey would be fine in a 10 gallon for probably 8-12 months. Depends on his growth rate. A 20 long would be a fine size to house an adult corn. Just make sure the snake has a couple different hides, a water dish big enough for him to soak in and something rough like a log or decoration that they can begin a shed on.
Let is know if your questions get answered and we want pics when you get him!