An accurate awning size can usually be found in your caravan's instruction manual under awning size or 'A' measurement however if you have lost you manual or if you want to ensure the measurement is accurate you can find out your 'A' measurement yourself. This measurement is to be taken ground to ground, following the awning rail along as demonstrated in the image below.
Porch Awning Measurements
The two measurements needed to buy the correct porch awning are the height from the ground to the tallest point of the awning rail, and the length of the most flat secion of the roof as demonstrated in the image below. You should also be aware of where a porch awning would fit to your caravan and whether there is possibility of it obstructing doors, hatches or windows.
Caravan Cover Measurements
To correctly measure for a caravan cover, measure the full length of the caravan at the longest point. Sizes in caravan covers are generally in feet rather than centimeters.