Hi Colm.
I have an Oxford Stormax. on my dakar which is taller than a standard 650 and it fits great with top box and panniers! I whack it on as soon as i have messed around with my lock, taken off my gloves and helmet with no problem. If i have been out on a longer run i balloon out the sides so it isn't touching the exhaust. Its also lined with a soft material which helps to stop scratches. Great in the rain, just leave a 2-3 inch gap from the ground so the rain water doesn't soak up into the cover! A large should do just fine! I paid £60 for it!
Good luck!
