Just occurred to me that I don't really know the criteria for choosing between, say, a #6 wood screw and a #8. My on-hand inventory tends to influence my choice, but that seems rather inelegant. So:
I'm in a situation where I'm going to use your basic Philips flat head wood screw, 1-1/2" long. I have both #6 and #8 from which to choose. I will do the appropriate countersink with a tapered bit [like the Fuller gizmo], and wax the screws.
Is there any what you might call "science" that influences the choice between #6 x 1-1/2" and #8 x 1-1/2"?
In case length plays a role, then same question for 1" and 2".