You'll need quite a large one. We've got 4 kids, and we put in everything except the tent (which goes in the boot) - so will fill the box with sleeping stuff, cooking stuff, clothes, folding chairs. Works well with the 600 litre box. Last time though I used a trailer....and it was even better as it meant daily access to what is in it is easier.
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.