Assume that basic 4wd box trailer would be available with a size somehere in the below range:
Size H L1 L2 L3 W1 W2
6 x 4 500mm 1800mm 1580mm 3380mm 1200mm 1720mm
7 x 4 500mm 2100mm 1560mm 3660mm 1200mm 1720mm
7 x 5 500mm 2100mm 1500mm 3600mm 1500mm 2020mm
8 x 5 500mm 2400mm 1500mm 3900mm 1500mm 2020mm
You could design with one these sizes in mind, or select a trailer size that best fits your design. However, keep in mind some key factors such as the towing capacity of the most likely towing vehicles (hilux, landcruiser, troopy etc) as well as the fact that 4wd trailers need to have slightly different designs, including longer draw bars and heavier suspensions.
There are lot’s of great resources on towing trailers in off-road conditions, so I’d recommend doing some further research in that space also.