Floorspace is more important than volume, especially for snakes that spend most of their time on the ground or in hides like BPs, so anything above 12" height is just decoration, or more versatile if you plan on keeping something else, and can keep the humidity/temps right. General guideline is length+width of the tank is good for a snake around that length, but probably more appropriate is about 1 foot of snake for 1 sq ft of cage. Of course it's a guideline, a large 5' active female going into a nice display terrarium might use and enjoy a 4'X2' cage, a shy slender 4'male breeder might do best in a dark and quiet 30x18 rack bin.