It looks as though you know how to find out the awning size using the piece of string method Sarah but for the benefit of any casual browsers who might wonder what is meant
We take a length of cord, wool, string etc from ground level at Point A and thread it through the awning rail B to ground level Point C.
The length of the string used when measured in centimetres will give a good idea of awning size when rounded up to the nearest 5cms.