Actually thats perfectly fine - young cornsnakes can be kept in things like plastic shoe boxes and be perfectly fine - a 20 even in a couple months would prob. be too big and stressful. a 10g is good. I personally wouldn't get a corn from a pet store - its overpriced and you won't know anything about the snake - I recommend getting one from a breeder!

If you have any more questions, check out this site - www.cornsnakes.com
Its an awesome forum that I'm a member of, and not only will you find a quality snake to purchase but everyone there is so friendly and willing to help!

Good luck getting a corn! They are so addicting - I was only going to have 1, but now I have a male and female and I'm in the market for 2 more!