Hey bro, glad to see your interested in a cube. Hope this helps sway your decision to purchasing a Cube.
Overall the cargo space is pretty fair inside. The back seats can be layed down to add space, and the utility container in the back can be removed to add even more space. You really wont need a roof rack unless of course you have +2 passengers in your cube. Here are some pics i just took of my cube inside. Its a little dirty but im getting ready to clean it.
http://i68.photobucket.com/albums/i1...s/DSC03401.jpg
this is the width on the base (gets a little wider as you move vertically.
http://i68.photobucket.com/albums/i1...s/DSC03402.jpg
without the cargo container
http://i68.photobucket.com/albums/i1...s/DSC03403.jpg
styrofoam taped to the bottom of the cargo conatiner to make it flush when the seats are folded down (came like this i didnt do it)
http://i68.photobucket.com/albums/i1...s/DSC03404.jpg
The MAT that i JUST FOUND when i removed the cargo container (didnt think i had one before now lol)
http://i68.photobucket.com/albums/i1...s/DSC03400.jpg
overall space to the back of the front seats
hope this helps you out bro. overall there is a lot of space , you really wont need a roof rack unless your a woman that needs 50 outfits for just a 3 day trip lol.