Solomon island tree boas stay under 4'. They could do well in a 20 gallon tall. They like cooler temps. If you look into gettin one of these MAKE SURE TO BUY ONE THAT'S ALREADY EATING MICE! they can be a NIGHTMARE toget started on mice, but other than that, they are AWESOME snakes! Most average 2-3' long. Info tree boas stay on the smaller side of 2-3' as adults as well.

Amazon tree boas CAN reach 6-7' but they tend to average 4-5.5' but even at this length, they are rather slender and don't really look the length. They are awesome snakes though, and come in LOTS of colors and patterns.