Yea you need to be looking for a big white box affair with windows and a door. On average about 5-7 metres long and with between two and four wheels preferably towards the centre. Also make sure its got a pointy bit on the front to fix it to your car.
Seriously though you are perhaps being to general. First thing I would do is go to some van showrooms see what takes your fancy (Make, style,interior set-up,price etc) you will then be in a better position to ask more direct questions ie is your vehicle suitable, single or twin axle etc etc.