Mike, don't know what the range is on the big birds but it isn't solely a function of length.
The absolute aft limit is the centre of pressure- a CG behind the CP will make the plane want to fly backwards. Even keeping it at the CP can be dicey, because ideally you want the CG to be ahead of it so that the plane will right itself if something knocks it around. The aft CG limit published in the POH is well ahead of the CP for this reason, which is one of the reasons the Cessna 172 is such a docile airplane.
The forward CG limit depends on the amount of elevator authority available and the load that can be placed on the airframe- the further forward it is, the more downforce needed on the tailplane to hold the nose up. This extra downforce will increase drag and increase the stall speed as more lift is needed, and will also put more stress on the fuselage between the wings & tail. The published forward limit is what the engineers figure the aircraft can safely fly at with a reasonable stall speed and plenty of elevator for pitch control (which is why the limit for the C172 moves forward as weight decreases).
Therefore, making the plane longer isn't the only way to increase the CG range. One can also increase the range by making the elevator bigger and/or further aft, and by reducing the payload. I'm sure the pros here know a few other tricks for extending the range as well.