Congrats.. corns are wonderful snakes. Adult size averages from 3.5' - 5'.

A 20 gallon tank would be a good size, yes. These guys do enjoy climbing from time to time, so if you like you can provide a branch or two. It's quite entertaining to watch.

Temps do not need to be in the same range as they do for a BP. Get your temps to around 88 on the warm side, and 80 on the cool. Don't let the temps fall below 75 degrees.

Make sure the hide spots are nice and small for him/her. Sometimes it helps to 'show' them where they are. Just pick them up and poke their head in the entryway.. usually the rest of the snake will follow.