OK, here's a ramble. I find this all a bit tricky to explain and make sure I've covered all the bases, so there's probably far too much information here. PM me if you want to talk it though.
We didn't order a standard size as none fitted at the time. Also the wrap around side panels weren't available, though we would have taken them if they had been.
Firstly, our *actual* measurements, then some sort of an attempt at an explanation of why we bought this size. As you look at the awning from broadside on to the van, the rear side panel is on your left, the front side panel is on your right and and the awning panel is facing you. When you are ordering, be very careful to use their notation though, as we tied ourselves in knots with them.
Both our side panels are 187cm deep (van to awning rail). The rear side panel is 185cm high at the van and 189cm high at the awning. I think we did it this way because the pole sits slightly lower at the van than at the awning. The front side panel is 185cm high at the van and 194cm at the awning (see reason below). The awning panel is 245cm wide, 189cm high on the left (rear of van) and 194cm high on the right (front of van).
The 'logic' for the odd shape is that you can drop the awning at the tail end of the van, but you can't drop it at the nose end of the van because the driver's door will scrape on the Windstopper pole. So we designed it to lift at the front and be level or drop if needed at the back. If you look at the picture with the rainbow earlier in this post you'll see what we do. This actually runs rain off better, by the way, than dropping both front and back. Martin's fixing may avoid this problem.
The vendor will put loops on the base and also loops on each of the vertical seams at around 12cm from the ground. We had a second set of loops put on the rear side panel too at around 25cm, for tensioning with the lowered awning, but not sure we've ever used them.
A bit over-engineered I think, especially as we regularly put the wrong side panels in the wrong place and don't notice!