not sure why they standardize on 2" increments, kinda like cranks. saw an add for a 3mil stroker the other day, have heard of 5mm and 6mm strokes too.
anyway, the shorter swingarm you can get away with the better. I personally have a +4 on my 4mm cub and it's a handfull. but, my bike is a duner first and foremost so, I'm not gonna budge on going over 4 inches. you can change gearing and paddles to help compromise the shorter arm.