Hi Martyn,
Apart from the height, porch awnings are measured in width.
The width isn't specific to different sized 'vans as the porch awning only slides along part of the awning rail (and is held, vertically, against the side of the 'van by the poles - so they are draughtier than full awnings).
You need, really, to decide the size of the porch to fit in with the windows and vents on your 'van. You can, of course, have the side of your awning running down the middle of a window but it's nicer not to!
It's also worth remembering that the size of the awning tends to be measured at ground level while the awning gets narrower towards the top (where your windows are).
Hope this helps.