do you have any experience in keeping snakes yet or is this your first snake? How big a cage are you able to provide for this snake? How old are you (in high school or adult), if you are planning on going to college, you will need to consider who will keep your snake while you are gone, as most colleges won't allow you to keep it in the dorms.
That being said, I would recommend a Honduran Milksnake. They CAN get as long as 6 feet, but that would be pretty unusual. Mine are great snakes, a bit jumpy like milks tend to be, but not so hyper that you cannot hold onto them. Remember that most baby snakes are very scared that you are going to eat them, and can bite and musk you to avoid that happening, so you might have to deal with that. Here is a link about milks, that lists their lengths and some other stuff. Remember that snakes are incredible escape artists, so make sure that the cage is EXTEMELY secure, especially the top.
Hope that helps and let us know what you decide.
phflame
milksnakes