While it may be on the small side for 5-6 people full time, it can be done. In addition there really is not another small/big car like it out there, and the fact that it has the Mazda DNA/driving feel, means that you can be practical, and still enjoy your driving experience. Having also had a 2002.5 Protege5 that I loved, (and I am always on the lookout for another one), the Mazda5 is a nice step up.
Think of it as a Protege5 with an extra row of front seats, and sliding doors.
The one thing I wonder about is how hard it would be to buckle a childseat into the third row, as that might be a little hard to reach. Maybe someone else can chime in on that, as I have never buckled a child seat into the third, just the second row.