Porch awnings are pretty much a standard size for height and will fit pretty much any van as they offer some adjustment for height when fitted.
As for size (length), these can obviously vary and are a personal choice on how much space you want and design of porch awning. A small awning for example, might sit well in relation to the door of the van, while a longer one might sit over an adjacent window and hinder it's use (there's pro's and con's of this).
I would wait until you have the van and decide what you want size wise and consider the positioning of the door with regards to close by windows etc., to help determine the best size awning for your needs.
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Paul
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I'd always err on the high side with height measurements. If you end up on a slope, you can easily lose six inches or a foot by the time you are a metre or two from the van.
We currently use one that fit the old motorhome - which was a foot higher than caravans. Although it always fits, the mrs doesnt like it because its a driveaway (though we never left it standing on its own) and therefore is more tent shaped than awning shaped and doesnt 'open up' enough for her liking.
We have a 260 porch awning (i.e. it's 2.6 metres wide) that's just enough space for a table, two comfortable chairs (not loungers), storage space for some odds & ends that we don't want cluttering up the van and a place to keep outdoor foot-wear & raincoats.
We can also hang up towels to dry in there, but they tend to be in the way.
We are also looking at getting a porch that is 260cm...anything smaller would go over the window. I'd wait till you get it and do a few measurements...if you order online it will only take a few days to get to you and if you have a local caravan shop...all the better.
That could be the overall height, not the awning rail height.
Best thing to do would be pitch the van on level ground and then measure from the ground up to the awning rail in the centre of the van.