If you measure the awning rail on the side of the caravan,and then add on the distance to the ground at each end(in centimetres)this will give you a measurement,which is known as the A frame length. This is the the awning size you require.
For example a Dorema size 10 is 875 to 900,size 11 900 to 925 centimetres.
Hope this helps
Cheers Richard