Have a look at http://service.isabella.net:591/isabella/uk/ameasurement/
If you put in the manufacturers name and the year of manufacture it will come up with a list of possible vans together eith theva measurement and the recommended awning size.
------------- Bill
For a licence dated 1997 or later you must add together the plated max weight of the caravan and trailer, if the total is 3500 or less you can tow it. You may even tow a caravan with a MAM greater than the cars unladen mass the restriction was removed in 2013
You could try using this link. Just enter the details of the caravan in the boxes and it should give you the size required providing that your caravan is listed.