As several people have mentioned, it depends on what you consider a "small" snake. Here are a few suggestions....
Cornsnake - can't go wrong here - big adults get to 4 feet.
Cal King - others have suggested this. I don't think they are the best first snake, as they tend to be a little nippy as babies. Some can reach 5-6 feet.
My problem with these two species is that they are everyone's first snake. Why not look at some of the other, slightly less common candidates which are at least as good (if not better) as a first snake?
Baird's Ratsnake - I think this species may be one of the best of all starter snakes. They are stunning to look at, easy to keep and the babies are big (like 15 inches - twice the size of a cornsnake baby) and easy to feed. The adults reach about the same size as a cornsnake. Babies aren't all that stunning, but the adults have a subtle beauty that few others snakes can rival, IMHO. They are also a little unique. Not everyone has a bairdi! Take a look at the ratsnake forum, there are always pics of people's bairds there.
Rosy Boa - small, even tempered, very handleable, easy to keep. Most rosies never exceed 3 feet. Come in a variety of colors, some of which are quite beautiful.
I have attached an Excel chart I made a while ago ranking good first snakes based on a series of criteria.
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Chris Harrison