my first own space was 4 x 8 meters, and we had 4-6, sometimes 8 people training. We even did a seminar with my Sensei with about 15 people. Rolling is very limited, but not impossible. Of course you have to work in groups if you want to roll (two groups length-wise).
Now we have a space about 10,5 x 9-11 meters, with two columns dividing it into two sections of 5x9 and 5x11 meters. This gives enough room to actually train with about 20 people and still being able to roll. More would fit for a seminar, but rolling is then limited to group work.
If you are interested, you can find pics on our website (go to multimedia, photos, dojos for the current dojo, and to seminars, May2002 Germany, first 3 pics for the old dojo.)
Oh, and weapons work was done in the garden in front of the old dojo, now it's in the dojo.
Have fun training...