You will need to check with your longarm person to see which direction they prefer the seam to run.
Some people want it to run vertical (top to bottom) and some want it to run horizontal (side to side).
I prefer horizontal, myself.
As far as the "extra fabric" goes... you will probably 5" ON EACH SIDE... so that means 10" total. This gives the longarmer room to attach the top and bottom edges to their leader, as well as having something to clamp on the sides.