I know it's a bit late but this link shows the towing law regarding caravan size:
http://www.direct.gov.uk/en/Motoring/DriverLicensing/CaravansTrailersCommercialVehicles/DG_192285
So a huge 2.55 meters wide 'for any towing vehicle, the length is however limited by the wieght of the towin vehicle, if can't be bothered to click the link here is a snippet:
Know your towing weight and width limits
Most standard cars have a maximum recommended towing weight. It's usually listed in the handbook or specification sheet. You can also find it on the vehicle identification number (VIN) plate on the car.
The VIN plate is usually under the bonnet or inside the driver's door. It will list a maximum vehicle weight and a maximum train weight. The maximum vehicle weight is what the fully loaded car weighs and the train weight is the maximum weight for the car plus a trailer. If your VIN plate doesn't list a train weight, you should not use your vehicle for towing.
Width and length
The width limit for any towing vehicle is 2.55 metres. The length limit is 7 metres for a trailer towed by a vehicle weighing up to 3.5 tonnes (3,500 kilograms).
Out of interest - 2.55 m = 8.36604 ft (decimal)
= 8' - 4 4/10" (feet, inches and tenths-of-an-inch)