For a male, you want a 4x2. Also remember boas are not pythons. They have a much slower metabolism so can be overfed very easily. Also they aren't supposed be round like a python. They should be kind of square like a loaf of bread. And also boas are not as stretchy as pythons when it comes to food so staying on the small side is better than too big or you might end up with regurgitation. In BIs, its not so common as they are garbage disposals but in a BC, too large can be a problem.