well for an adult male that gets to say 4.5-5ft (larger end) it still gets 6ft sq floor space. These are the kind of sizes most breeders would use, and how much the snake utilizes. They aren't monster sized snakes (all relative I suppose) but yes there may be some giant 7 ft individuals, but columbians tend to be even smaller than brazilians on averages, 5ft maybe for a female (larger than males).
Remember these are semi arboreal snakes so a bit of height can be good for them also.
Personally I like the 4x2x2, but as your in the UK if you don't mind them, the really useful boxes have a size that they do great in, and are great for humidity.