From Yikes: <<The Large size cover easily goes over the side cases and stretches down nearly to the ground. There is a little slack over the handlebars between my upright mirrors, but that should be taken up when I put my windshield on (like I see your bike has). A café style bike with no cases, windshield and with bar end mirrors could probably get by with a Medium size, but I’d recommend the Large.>>
+1 on that! Wish I had seen this thread two weeks ago. On 24 May I went to Bob's BMW (drove my Bug rather than the bike because it needed exercise) and bought a Guardian medium more or less on impulse. It fits over my Shoei Sport Fairing (somewhere around an RS in size) and upright mirrors, and is a fairly tight fit over the BMW touring cases. Problem is, there is little or no margin for expansion.
When I tried the fit on the bike, my inclination was to return the medium for exchange because I hope to change to a slightly higher windscreen sometime soon. I took it out onto the deck of our house to refold it for the box, and being a conscientious customer, put it on the picnic table to keep it out of the dirt. To my horror, the red paint on the table had oxidized, and it left some smudges on the silvered portion of the cover. I tried brushing the stains off, but had no luck.
I'm not comfortable trying to return the medium in that condition to Bob's, so I'll make do with it for the present. If I change windscreens, I'm likely to buy a large cover, in which case I might make available a slightly used medium for a very reasonable price. Stay tuned.