Bobbie,
My notes show the distance between the outside of the ties of Atlas O O-72 is 6' 3". This was the measurement of a half circle I laid out a while back. I think you would want at least two inches from the end of the tie to the edge of the table, so that would make the table width at least 6' 7". If you have an edge of the table against a wall, you will need a couple more inches especially if you run Big Boys. Maybe someone with RRTrack available can come up with a better number.
Ron