They're not morphs like ball pythons or leopard geckos. They are different subspecies and each varies slighty. Carpets can get up to 10 feet but 7 or 8 is more common. The smallest is the irian jaya which stays in the 4-5 foot range. Do some research and find what you like best. I have my JCP in a 4x2x1 from repti-racks. Many people will tell you that carpets are really arboreal, but thats not entirely true. A foot high enclosure if fine for an adult carpet.

http://www.acreptiles.com/carpet_python_care.htm

Go there. They have beautiful snakes there and have a great overview of the different carpets.