Regarding Frank's description of density and tallness; That is, where a tallness/density value of '1' = 100%.
I think as you go past '1' you are, in effect, stretching the dimensions of the stone in tallness values, and overlapping the density.
A density of '1' is pretty much complete coverage(you'll find it isn't total ground coverage, though) and,
a tallness of '1' is a stone of roughly the same measurements on each axis, like it would fit inside a sphere with its extremities touching the inside(approximately).
The default 0.5 tallness is pretty natural for a stone on the ground in real life, half is most probably buried underground.