When I ordered my speakers in September of 2018, I had this same conversation with Dr. West.
My listening room at the time was almost identical to yours; 11 x 18 x 8.
I was torn between the U545 or the M645.
Dr. West recommended the M645 given the size of the room. I would think sizing the panel is the most important, followed by the frame material.
Right now after moving, I’m in a smaller listening room and the 645s still work well.