Scarlet kingsnakes... Which are actually a milk snake, usually stay under 20". I know a couple people who have had 2 foot specimens though. To be honest, they need a small enclosure, with them it's not "the bigger the better", because scarlet kings are shy animals and having a larger enclosure will throw them off and they will want to hide more than usual.
California kings get to about 3-4 feet on average. If they stay under 3 feet their whole lives their growth is stunted. 
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Mike
KingPin Reptiles Inc.
www.freewebs.com/mikesnake
www.captivebredforum.com