For the overhang, 2" works good for me. I just made the base 4" smaller all around - the recommended table top dimensions seem pretty good.
Also, when you look at the plans you'll notice 8.5" boards that double the thickness of the legs at the bottom. These locate the bottom shelf at 8.5" (plus the 2x4 height) above the floor. Too high in my opinion. Just high enough to get your toes or a vacuum hose under would be better.
First thing I noticed when I finished the tables was the wasted space under the shelf.