It's a flying squirrel. I had to chase one out of our cabin one time, darn thing tried to climb the bathroom wall to get out the window and it fell in the toilet, what a mess. Anyway I got several very good close-up looks at it, and that is the very first thing that came to mind. Same grey color, same large eyes (fishers are much darker brown - black). This looks like a big one, but when they run or jump (into a toilet), the skin flaps make them look much larger. It looks like this one has just landed and it's skin is flopping all over thus making it look bigger.
Cool pic.