There are legal limits on the tray size you can put on a vehicle. As a guide, we have included a table (tray sizes, click here) for commonly used vehicles. You can calculate the maximum size of a tray using the following formula:
A x 0.6 + B-100 = Maximum legal length in mm
Where A is distance between the wheel axels in mm and B is the distance from rear of cab to centre of rear wheel in mm. Obviously common sense should prevail, irrespective of the legal calculation, the tray over hang should not be excessive (typical over hang is 150 to 300mm only).
If you are having a tow bar, it is better have normal length trays (not very long trays even though they are legal) due to accessibility reasons.