If it helps so that you don't hire something bigger (or smaller)than you need….
Transit/Sprinters internal load spaces are:
8ft long (SWB)
10ft long(MWB)
@12-13 feet long (LWB)
Confusingly hire co.s may consider a Medium wheel base to be 'Long' and a Long Wheelbase to be 'Extra Long' so try and get clarification. They'll all be about 4ft between the wheel arches and five wide everywhere else. At medium or long wheelbase the body will be 5 – 6ft tall too.
Some places might fob you off with a Traffic, which class for class is smaller than a transit
The biggest volume you can get on a regular license is with a Luton, with which you'll gain an extra 2ft of width a couple of feet of hight. The bodies are custom built so the length depends on the vendor – I use ones that are 14ft long but they can be as little as 10ft.
The weight of the body and tail lift is so much though that some Lutons can only carry around 750kg, half what you could carry in MWB panel van
You can drive a 7.5 tonner on your license, but not in any capacity that can be linked to work. So moving house is OK, but you can get caught out otherwise (like your pal paying you £50 to help him move house – that gets you both busted!).