Whats a good reptile for 30 gallon tank?

I have a 30 gallon tank and i would like to get a reptile that would fit perfect for and not have to upsize the tank as it grows. I am not intrested in geckos. more along snakes or lizards? And if you would get a retile that grows big for example a iguana. would it not grow and just fit the size to its surrondings?