It can be disconcerting going from a larger space to a smaller space. I went to a mini-seminar my dojo and another dojo had. The seminar was located at the other school and they have a much smaller space than us. It is long and narrow, too. The hardest part for me was taking my ukemi towards the walls. I'm used to more room so I take a little father to roll.
The key really is that everyone needs to be aware of their surroundings. At one of our branch dojos, we train in small aerobics room and our mat size is limited. And 8 on the mat was really tight. Having people throw opposite directions, really helped alot, and being aware of when the other people where throwing was critical to avoid collisions.