I have a '05 outback and am buying a fat sac tomorrow.

Does any one know what exact size I need to buy to fit in the rear of the boat if I raise the rear seat up to the top section?

Also are there any special sacs I need for the front of the boat.
I have been told I need some in the front also.
I am hoping to find a size I can put inside the front seats so I do not loose any cabin space.