Help!!! Groundsheet Size?

Hi Everyone

Ive recently returned to caravaning after a 25 year absence - a historic vanner/rallier when I was a kid and finally talked the wife into letting me get a van and sharing the fun times I had with my little boys.  

Anyway.......more to the point.

Ive thankfully had included in my amazing van deal an Isabella Capri Lux Awning which from checking the size requirement of my van is a 975 size.

My question is what size ground sheet do I need to ask to be cut to measure - been looking at the bolon which is  cut to measure at my local dealer.

Can not seem to find anything online and forgot to measure last weekend while it was up.....DOH!

Need to order soon as Im off to Spain in what will be only the 4th time out in her.  Hey, I sat in the back for long enough as a kid, why not try it from the front.

Appreciate any advise or help that you can give - and look forward to getting to know a few of you on here.

Middle Middle Middle.......