Unless there is something very unusual about the shape of your caravan you just need to measure the length of the body at its longest (normally just below the windows) with a tape measure. Some awning makers might ask you for the maximun height as well, but not normally. Unless very specifically made for one caravan, most awnings will fit a small range of sizes - for example 15 to 16 feet, or whatever. I have purchased awnings for two caravans now and find that the simple body length measurement is fine.