I bought mine big enough to come down the sides, just around the bottom of the window seal and long enough to cover the top of the bed hatch in front in back. I don't have the new seal kit for my roo, so I added some extra insurance and tied a 2nd tarp (another $4 on sale) to the large one to cover the bottom hinge of the front bed door so I don't get any small leaks in there.
One thing that I did learn was to not every buy those rubber type bungees with the metal S-shaped hooks on them. The rubber weathers very quickly and I found myself having to replace many of them or double them up so they didn't break on me. End result...I trashed all of them and replaced them with the nylon wrapped ones with the plastic hooks. I think I paid like $10 for a round tube of 10 cords with different sizes and colors. Worked like a charm. I Didn't have to rewash the black rubber marks off of the camper again either, which was a heck of a difficult job the first time around.
Where we've been:
2000 Rockwood Roo M-18 Hybrid
2006 Dodge Ram 2500 Auto 5.9 Diesel
Gone but not forgotten -1986 Rockwood 1003 PUP