Allen,
The idea, generally, is to balance the flap area. If you use equal panel wings, then it's easy. The flaps are the same. If you use any asymmetry then it can get more complicated. If you have full span flaps, then the outboard flaps has to have a larger tip chord to balance the area with the inboard flap; the idea being that both flaps have the same area.
Some designers give the outboard flap a bit more area than the inboard flap. Not a lot, but maybe 2 or 3% bigger. This allows you to carry more tip weight.