I'm looking to get a cornsnake, but I have read that they can get up to 5 feet. I really want one but just don't want one that big. I have been told that they will only grow as big as their cage will allow. Is this true or just a myth? I have a 20 gallon tank, but not looking for one that's going to be as big as me. Thanks for any help.

No it's not true. if you don't want a snake that gets that big get a different snake 5 feet really isn't that big they are tiny snakes and are long but not big in width. 20 gallons is more than enough for a full grown cornsnake.

Most grow to three feet not 5 5 is the very rare largest size the grow to not the average and if you can't handle a 3-5 ft snake you might want tyo look into geckos or some other reptile because they don;t get much smaller.

I mean think about it if animals grew only to be as big as their enclosure everyone could own and anaconda. If a cornsnake or even a ball python were as big as you when it was fully grown you'd have to be ridiculously slim and very tiny.

http://www.aqualandpetsplus.com/Snake,%20Corn.htm
As you can see that's a kid holding a full grown corn snake so again really how is 5 feet going to be a snake as big as you?

Badkittyamy is correct. Contrary to popular belief, snakes will not stay the correct size for their inclosure. If you keep a snake in a tank that is too small and it cannot stretch out properly, then you are putting it's health at risk. They need to be able to stretch out to a certain extent to digest and breath normally.

My old cornsnake was about 4 feet when he stopped growing so that might give you an idea. You can estimate anywhere from 3 to 4 feet. 5 feet is rare.