You need to measure from the ground around awning channel and down to the ground again and then you need a size awning that covers this length.If you go slightly larger you can always peg the sides out, but smaller you are stuck.
I am just wondering if your van is a swift.We had an 1997 swift jura and its height was the same as the new vans.Some of the older vans i found out were higher than some of that day.