I've enlarged 6x6 to 24" and found the image to hold together very well. I shoot colour. I'm not ashamed of the medium and don't care if the grain begins to show -- actually I want the grain to emerge, otherwise I'd be inclined to shoot digital or inject a digital step. I do care that the image looks as I'd like, that it doesn't fall apart and most importantly that image requires being that large in the first place.
If doing the math is important, then it might be worth looking into the resolving power of the Mamiya 6 and 7 lenses. They tend to be far better than all but the very best LF lenses (and as good or slightly better than the very best lf lenses), this combined with not having to stop down as much to achieve the same DOF and smaller distances of projection for similar enlargements tends to nullify or make less significant the differences in film size.
Photography isn't done with a calculator and 4x5 tends to be better than 6x6 from a mamiya 6, but not substantially. I wouldn't choose lf over my mamiya for capacity of enlargement alone as it just isn't worth the effort.