well for adult corn snakes, a 20 gallon long tank would be good.
well tame.......actually corn snakes are naturally quite tame. They don't really bite or anything like that. Even if they bite it doesn't hurt......but anyway, to answer your question and some suggestions:
yes they are quite tame and you can tell if they are tame by asking the pet store to show you the snake. If it bites before the petshop guy/gal picks it up, it isn't tamed yet (but this rarely happens.....) or if it's on your hand already, try holding a figure in front of the snake horizontally and see if it bites....if it does.....it's not tamed down yet.....
to tame a snake down.....put more time in it.....hold it as often as possible, BUT not after it has eaten (leave it alone for at least 48 hours before picking it up). Also do NOT disturb it for a few days when you do buy a snake and put it in it's enclosure. Leave it in the tank for at least 3-4 days before picking it up. As new environment is already stressful, picking it up creates more stress...so remember not to pic them up during these periods. During the 'holding days' you can slowly increase the amount of time you are holding it. Say 5 min for the first few times and then 10 mins next few times and then so on.....but then there is a thing. Since all snakes has it's own personality, it is possible that the snake will not grow to be a tame snake.....but this usally doesn't happen but there are cases like that...
one more advice is that you should buy your snakes from breeders, as
1) they are usally cheaper
2) they are usally healthier
3) they are better looking
4) you get guarentee from breeder
you can also ask the breeder to get a snake that he/she thinks is quite tame from the start.
here are some breeders for your information (in no particular order):
www.corn-utopia.com (Bill and Kathy Loves)
www.serpenco.com (Rich Zuchowski)
www.cornsnakes.net (South Mountain Reptiles)
www.gherp.com (Glads Herp)
all these breeders have great corn snakes and VERY nice people!
also if you are new to this, go get the "Corn Snake Manuel" from Bill and Kathy Loves, it's a book that all corn snakes owner should have.
Hope this helped...
Good Luck and Happy Herping!