So is the "muscles" option, like, the muscle mass that would increase the diameter of the limbs by that much (or depth if it also goes for non-limb muscles)? Is it just for the arms, or for all muscles (within reason--I don't want a tongue that's even another quarter-inch thicker D

?
If it's for all muscles, half-to-full-inch to them and the rest to arm length.
If it's just for arm muscles...I'd probably do that same thing. Arm strength plus orangutan arms all day erryday; my wang's fine as it is.