The awning will have a size range, example my awning size is 880 - 910cm it was purchased for my last caravan which had a size of 903cm.
My new caravan has an awning size of 920cm and it just fits.
Some awning sizes will be 800 - 850cm, 850 - 900cm.
The awning sizes are actually in centimetres and not millimetres which is important when working it out. However, the simplest way to work it out is convert centimetres to feet and inches then you have a better idea of how much excess material there is. So, 900 centimetres minus 880 centimetres equals 20 centimetres which converted from metric to imperial works out to about 8 inches therefore if you have 8 inches excess material then you need to lose 4 inches at either end and this is the bit where you can ask yourself if the awning is too big or not. Personally, I think it is unless you want to put the caravan on ramps every time to gain that extra 4 inches.
I have just bought a Walker Weekender (ground-to-ground canopy) from Jeff Bowen Awnings. They give you details of how to measure your awning, you advise them of the measurements, and Walkers make the awning to size.
Bertie.