Size
Three sizes cover nearly every job. A 20ft is 20' L × 8' W × 8'6" H and holds 1,172 cu ft, and it fits a standard driveway. A 40ft doubles the length for 2,390 cu ft at the same 8'6" exterior height. The 40ft High Cube keeps that footprint and adds a foot on top: 2,694 cu ft, with 8'10" of interior headroom instead of 7'10", which is what matters if you plan to shelve it or finish the inside. Door width is 7'8" on all three, so what you are really choosing between is length and headroom.
Compare the three sizes