Didn't think they were that tiny, but O.K
Here is a pic OP, it will give you some idea.
http://regretfulmorning.com/wp-content/uploads/2008/08/passat-wagon.jpg If you want bigger than that then I would suggest the Volvo V70 even more as those things are huge in the back for the size of the vehicle. Volvo don't mind compromising aesthetics for maximum practicality. The very sharp "cutoff" rear on them gives you a lot more usable space in the back. I seem to remember Jeremy Clarkson putting a large sofa in the back of one to demonstrate this on Topgear (That or it was the older 850 model).
The 2.4 Volvo is surprisingly economical for its size too.
Depends whether you are going to use it with the seats folded down or not.
Here is a pic OP, it will give you some idea.
http://imageshack.us/photo/my-images/43/dscf5095o.jpg/ The Passat keeps a lot of room for the rear passengers so if you are going to fold the seats down then you will have a load of room, but with the seats up it has a bit less than some other wagons as Philip pointed out.
If you want bigger than that then I would suggest the Volvo V70 even more as those things are huge in the back for the size of the vehicle. Volvo don't mind compromising aesthetics for maximum practicality. The very sharp "cutoff" rear on them gives you a lot more usable space in the back. I seem to remember Jeremy Clarkson putting a large sofa in the back of one to demonstrate this on Topgear (That or it was the older 850 model).
The 2.4 Volvo is surprisingly economical for its size too.