They don't generally get 6 feet. The males average 4-5 feet and around 1000 grams +/- as adults. The females do get a bit larger, and 6' is possible and has happened, but it is rare.
They need a 3x2x2 foot cage simply because they are chondros, they are truly arboreal, and they need that side to side ability to thermoregulate.
Hope you get the book for Christmas! It really is a great read!