This orientation question seems fairly simple. Make your robot wide if you plan on doing any kind of serious ball gathering. Unless there are other mechanisms that reqire a long orientation, I think wide is the way to go for a maximum frontal opening and ball gathering space. Remember the diameter of the balls is 9". Based on the 6" bumper rule, the maximum opening would be 16" for a long robot and 26" for a wide robot.
However, bumpers have to extend into the corners and can be up to 3 1/2" thick. This sets up the possibility that part of the 6" for the front bumpers will overlap the side bumper and each front bumper will actually cover only 2 1/2" of the front side leaving a possible 33" opening for a wide orientation and 23" opening for long orientation.
Based on Team Update #2 and the drawing with the trailer and the robot, I think this is not FIRST's intention and that the six inches must be part of the actual robot surface. I plan to ask on Q&A.
Ooops, it looks like some one got to it before me. The bumper must be touching the robot to be considered part of the 6".