A 4x3 trailer doubled in Hight fits most of the gear in and the cool box and bag full of cloths in the boot, the trailer holds about 800 litres which is the same as a zafira boot.plus the boot on the car is about 350 litre plus a few pillows on the kids knees if they want them?
I have a Yaris 1.33l 5 Door with Toyota Roof Bars and a Karrite Odessy Roofbox which has a capacity of 380 litres. Holds the cooker, Weber Go-Anywhere BBQ, Tent Carpet, Windbreak and poles and other lightlight stuff, such as shoes, kettle etc...You have to watch the weight which should not exceed 50 kilos including the roof box ( 13 kilos ). I have to say however, that there are only 2 of us in the Yaris so we have plenty of room ( I like to keep the rear window clear, so our stuff only gets loaded to seat height ) but I'm sure the roof box will give you the extra capacilty you need.