A 20 long would be big enough to house a corn its entire life, so a hatchling would do fine in that as well. You could go as big as your budget allows. A 40 long makes a nice display, as does a 55. The larger the tank the more hides you can provide and the more likely your snake will explore and find the one he likes best. In the smaller tank, one or two hides are all that you can put so your snake doesn't really have that much of a choice. That's my two cents. I'd have each of mine in a 40 long if I could afford it and space allowed, but I'm going to build a rack for them all soon.