How big a notch to use...

I am about to bag my 86 C10, and wuz wundering how big a notch some of you guys use. I heard of one guy using a 6" round notch and that wuz just fine, but wanted yall's input.

oh yeah, i'm gonna be usin 15" rims w/ 235/60/r15 tires, since i still want some rubber to the ground.