I would also go for the 890-910 cms. awning as for reasons as 'madasaman' said. My official awning size is 886 cms. but I bought a 900 cms. Dorema awning and it fits just right on level ground. If the awning is baggy due to uneven ground you can always partly drive the caravan up a pair of ramps to take up any slack but if the awning is too short then there's no solution except exchange for the next size up.
With an awning rail that is 890cm most awning manufacturers would suggest a 900cm awning.
------------- 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