I don't think you can have speakers that are "acoustically" too big for a room -- you just have to think about the physical aspects. As an example, M80s are pretty deep so the fronts of the speakers are going to end up over 2 feet away from the wall, and maybe a bit more once you find the spacing that sounds best. If that is so far into the room that they are too close to you or you can't walk around the room that might be a problem, but otherwise no issues.
Oh yeah, big speakers are a pain in the butt to carry around, particularly going up and down stairs. Ask me how I know that
EDIT -- hold on, your room is 11x18 feet. There are people on the boards running M80s in dorm-sized rooms -- one guy even has an EP600 in there too.
I thought you were talking about a SMALL room
