I like my runners to hang off the ends of the table, but no one sits at the ends... I would make it 6' (72"). When 0 or 1 of the leaves are installed, it will hang off the ends, and when the table is fully extended it can 'float' in the middle.
As to your 'trial and error' comment, I would get a sheet and fold it to different sizes to see what you like. As far as a round table, you need to either make it big enough so the plates sit fully on it, or make it small enough that they don't. Same thing with a runner.