Most large constrictors will do fine in a 6x3x18" enclosure. That's going to be a pretty big investment. This is what I would do provided she is of relatively small size (adult BP)

CB70 sized tub / enclosure for now
From there I would switch to something like the Iris VE-175 tub (make sure the tubs are properly secured)
And then after that I would make the jump to an adult sized PVC enclosure.