It's not a bus, but I just wanted to demonstrate that there seems to be no limit on size.
Haul trucks are basically massive dump trucks used in mining operations. They, of course, consume massive amounts of fuel, and so increasing effiency marginally can save massive amounts of fuel. Already, most new haul trucks are electrically powered, like diesel locomotives. Their diesel engines power generators that power the electric motors. This is more efficient than traditional drive trains, and provides much more torque for moving such gigantic loads.