G'day & Welcome John.
I agree with earlier comments, a walkway style soft floor is probably going to work best for you, though depending on the age of your children a hardfloor and extra room might do it if the older kids are keen to sleep in their own tent/swag.
The real advantage of the walkway soft floor campers is the storage they provide; enough for the clothes, bedding and food that a large family requires. I'd recommend having a look at Complete Campsite and Trackabout.
I would also advise you to stay away from getting the biggest tent possible; it seems like a great idea until you try to put it up, on your own, and then away or try to fit onto a caravan park site. I believe you can fit into a 10' tent easily, and maybe stretch to 12' if you must. The bigger the tent the heavier it is.
