Spotteds actually get a bit bigger than Childrens. Though I think either would be a good choice, as they don't get very bulky at all, and even a huge female Spotted will probably top out at 4 feet. They say they can get up to 5, but that's probably about as common as the 14 foot boas you hear about.
If even that is too big, then I say maybe a Rosy boa or sand boa.
Any particular reason you have such a small size limit? Bigger snakes just make you nervous, space constraints, ect.?