what is the biggest size [ ie max weight ] a 7.5 tonner could tow ?
splinters:
what is the biggest size [ ie max weight ] a 7.5 tonner could tow ?
whatever it says as “gross train weight” on the plate minus the 7.5 tonne GVW of the truck 
it also depends on the drivers licence
( I couldn’t find a real answer 
)
With regards to weight, Denis F is right. To give an example; if the Train weight is 11000 Kg and the permitted gross weight is 7490 then the max permitted trailer (including load) would be 3510 kg.
Don’t forget that the trailer will need to be plated and tested if its’ UNLADEN weight exceeds 1020 kg. In the case of 2 axle car transporter trailers this is unlikely. However, if the trailer is a two axle box it is most probable that you will need plating and testing. There is an exemption from plating and testing if the trailer only requires parking and overrun brakes (where the permitted gross weight does not exceed 3500kg).
Drivers licence; if you passed your car test prior to 01/01/1997 then you will have C1 (vehicles above 3500kg and not exceeding 7500kg) you will also have E (a trailer). BE REALLY CAREFUL, the ‘E’ bit will have a category 107 restriction; the train weight of the combination may not exceed 8250 kg, in other words you can only drive a 7.5 t vehicle towing a 750kg (permitted maximum weight) trailer. If you passed your car test after 01/01/1997 then you will need to take a C1 test anyway. You then need to take an E (trailer) test then you can go upto the maximum permitted weight, which is 12000kg. If you’ve passed an old class one hgv (or the new C+E) test then you can drive C1+E upto the 12000kg limit. A final point is that the maximum authorised mass of the trailer cannot exceed the UNLADEN weight of the drawing vehicle.
If you’re worried about dimensions; then the maximum drawbar trailer length is 12m (where the trailer has at least 4 wheels) and the maximum trailer width is 2.55m. This increases to 2.6m if the trailer is refridgerated. The maximum overall length of the combination (prime mover + trailer) is 18.75m