I don't think there are any "official" guidelines like that anywhere. It sounds like you're doing the right thing though by making different swatches and seeing which one you like the best. I made a lap afghan last Christmas using 2 strands of Homespun and I used a size 17 needle for it.
It all depends on how close together you want the stitches to be. You could try making your samples a little larger, just to get a better idea of what an afghan would look like, but other than that, it looks like you're on the right track!!!!!