Most driveaway awnings will come with standard and tall sizes. As an example, my fairford, long wheelbase van conversion takes the tall, as did my previous coachbuilt. A vw transporter is likely to be standard.
You want a snug fit at the joining section and will only get this with the correct height. The sides, where it joins the van.....not so snug, but bear in mind this is only the joining section.
Using throw over straps are also useful to secure awning, but best to fit to awning rail with filet strip and figure 8 connections for ease of removal and reattchment.
This works for us, but others might prefer something else
The main consideration for which awning to purchase is the height of your awning rail/wind out canopy or roof of your vehicle that the awning is to be attached to (Please note when looking at vehicle heights, bear in mind manufacturers literature states overall height of the vehicle including skylights/aerials etc., not roof height or rail height).
Outdoor Revolution awnings specify what heights the awning is suitable for, see each awning on our website for more info. Simply measure from the ground to the awning rail or roof and this will tell you which awning to select. – Look out for wider awnings if you are wishing to open sliding doors inside the awning tunnel, but remember don’t go too wide because the tunnel will be too wide for your vehicle, (very rare but a consideration non the less).
You can find a helpful guide here:
https://www.capitaloutdoors.co.uk/blogs/outdoor-blog/outdoor-revolution-drive-away-awnings-comparison-chart