Both the racks I've built have been 5'x10' which are great sizes for 3-4 people on top. The rack I built for The Camo Beast worked great but it was permanently bolted to the roof and the chairs screwed into 2" pipe collars. The rack for The Reaper bolts onto a luggage type rack that we built on the top so that it is removable. I much prefer the removable rack so that I don't have an air drag on top all the time.
If your suburban is only a half ton you will want to watch how big you make the rack, even expanded metal catches a ton of wind and in corners wind can do some wild things in a corner. That's why I always buy 3/4 ton suburbans, the extra springs help when the wind starts pushing you around.
I used the hold downs from the luggage rack on The Reaper and they work fine but if I redo it I will lower the headliner and run bolts through with LARGE washers like I did with The Beast.