Boy, my space isn't as horrid as what you folks have dealt with! Gives me great hope.
I've got a decent size mat, but it is long and narrow, and has wall abutting on one side (making that part of the mat less usable) and viewing area on the other (have to watch for visitors not to kick them in the face). The measurements are 5 meters by 8 meters plus an extra two tatami at the far end next to the tokonoma that has yet to be properly installed. The biggest problem is the wall. Yesterday I held my first class and was dumbstruck by the configuration and realized that unless we did lots of pins, actual throws along the short side would be impossible and we'd have to do line throws along the long side, but might only be able to get one line in instead of two. My current students are all at the point where they have enough control over their rolls whereas not to bash into the walls, but new students are going to have trouble with this.
Can't believe I made such a big mistake on my vision for this space, and wish I'd listened to my student who kept telling me "meters, not feet" with the mat.