I'm currently on the hunt for a 9 x 9, reasons for it are it's a good size for 2 people and can easily be set up by them, you can stand up in it and mainly as I have a Wolf it can connect giving me an easy storage area / power for lighting etc.
The crew shelters are ok but low and require a vehicle to attach too.
12 x 12 nice size for a family, heavy to set up best with 3 people, takes up a lot of room and can be cold as it's a large area to warm up.
Poncho perfect if on your own and don't mind using your bivi bag.