In reply to gali:
Always thought it's based solely on height and length? Been several times in a transit, always just booked on as a private vehicle.
Went by Chunnel once, filled in the form accurately and it said my SWB Low Roof Transit was car dimensions so booked a car ticket. Argued for a while at check in, got them to measure it, and it came in (as I knew it would!) under the car length and height sizes. Result!