Ha! Same boat as you are in again LOL!
I want my son's royal in the Pro-Line that the carpet is currently in. The carpet I have is some type of coastal cross with the mother at about 10 feet and the father about 8 so I'm expecting an 8 to 10 foot animal and it will need some ceiling as well as floor space.
This brings us back on topic.
Another way of "Knowing" when your snake can move to a larger enclosure is if it is clearly outgrowing it's current one.