corn snakes only ever need a 20 gallon long, but will appreciate any size enclosure you give them. I know someone who gave their corn snake an 80 gallon enclosure, and he just uses the heck out of it and explores like crazy. They get to about 5 feet, but they are so much thinner you wouldn't know it. ( http://www.aqualandpetsplus.com/Snake,167.jpg
is as big as you'll generally ever see them get. If that's still too big for your taste, get a male, males generally don't go much further than 3 feet)
If you use an aquarium make sure the lid has a lock on it.
Feed them frozen rats or mice about once a week, they are not as picky as ball pythons so they don't need all the difficult time it takes to switch them over from live. Appropriate sized feeding means the rodent is as big around (or sliiightly bigger, less than 25% bigger) as the snake. Corn snakes are small enough to eat mice their entire lives so you don't have to switch to rats.
Temps--70 cool side-85 warm side. One basking spot that makes it to 90 F,
Humidity, normal ambient household humidity is perfect.
Make sure they have a water dish. Substrate can be virtually anything except pine which is toxic. Aspen, newspaper, cocohusk, etc.
And give them places to hide. When they are ready to shed, put some moss or damp paper towel inside one of the hides to create a humid hide. You can give them a humid hide all the time, but they rarely use it other than shedding to be honest.
Corns are docile, but babies are tiny and scared--and known to be nippy! But they are so small they can't even break the skin most of the time, and it feels like someone scratching at you with a toothpick. Adults settle down fast. Pick a juvenile if you want to skip that baby phase, heh.
My dream corn snakes are from this breeder: http://www.poppycorns.com/index.html