The relationship between geometrical properties of stellar disks (the flatness and the truncation radius) and disk kinematics are considered for edge-on galaxies. It is shown that the observed thickness of the disks agrees with the condition of marginal local gravitational stability. As a consequence, those galaxies whose disks are thinner should harbor more massive dark halos. A correlation between the de-projected central brightness of disks and their flatness is found (low surface brightness disks tend to be the thinnest ones). We also show that the positions of the observed photometrically determined truncation radii Rcut of stellar disks agree well with the hypothesis of marginal local gravitational stability of gaseous proto-disks at R = Rcut.