What size cover works with the Interstate?
A big one.
Generally, the same cover as for a Goldwing.
Most sellers have fitment charts. (Though Valks are dropping off them these days)
Covers, intended for outdoors, need to be snug, but my lifetime experience with them is they are a big PITA to get on and off, with the elastic bottom catching on every exhaust, fender, peg, rack, signal, handlebar, mirror and so on. They have to be
struggled on and off. So bigger is better than smaller/tighter.
I believe you can get big (X)XXL covers for Interstate that have (partial) zippers in back to make them easier on/off, but I've always wondered if a zipper is a scratch hazard, and/or fully waterproof.
I had high winds take my full-covered CB750 with big Vetter fairing and sissy bar right off the center stand to it's side (leaking gas and oil), but that was out in an exposed parking lot next to base BOQ on the flight line. Happened several times; the cover turned it into a sailboat.
But I've never heard of a Valk blowing over; the fat bastard.

Now this company is in my county, and they sell (expensive) unique, tight stretchy ones, but I've never seen one or read any opinions. But they look interesting.
http://www.gezagear.com/
More like a condom than a cover (but without the lube).
