What you need to find first is your 'A' measurement, this is the length of your awning rail from ground to ground.
How to do it:
Get a long ball of string peg it at one end of your van under the end of your awning rail, then feed the string through the awning rail to the ground on the other end and mark the string at this point with a knot. Take the string out and measure between peg and knot - this is your 'A' measurement.
Most awnings will have a 'A' measurement over 40 cm e.g. 875-915 so long as your measurement is within this figure it should fit
------------- Caravanning is a way of getting a cheap holiday out of an expensive hobby
some label them 12, 13, 14 etc but others are labelled 825-875cm etc
Watch out for this one as the size (12, 13, 14, etc) bears no relationship to the 'A' measurement (825-875) between manufacturers. 'A' measurement is always correct.
------------- Caravanning is a way of getting a cheap holiday out of an expensive hobby
If you go to www.isabella.net and input your caravan make, model and year it will tell you what size you need. As previous replies have said, most makes measure in 25cm increments (it should fit if your 'A measurement' is say 10cm either side of this size) but companies such as Dorema go size 1,2,3 ect. If you look on their website it will tell you what 'size 3' is in the centremeter size - ie I think size 3 is 750cm.