Our two-fold purpose was (i) to quantitatively identify characteristic vectors, i.e., specific combinations of joint angles of fingers, that dominate the posture variation of precision grips in the configuration space (the space with joint variables of the fingers in the hand), and (ii) to investigate linear correlations between the postural variation and the object size using the parameters of the characteristic vectors. Experiments involving 14 participants measured the grip postures of the participants who were asked to grasp eight cylindrical objects of different diameters using six designated precision grips (spherical, tripod, pinch, quad-pinch, tri-pinch, and mini-pinch). Regression analysis showed that within each precision grip, the postures changed gradually in a unique linear direction in the configuration space with an increasing object size relative to the hand size. Quantitative models of the precision grips were established that could be used to reproduce the precision grip postures for grasping various object sizes by adults of various hand sizes.