We investigated the contribution of haptic and visual information about object size to both perception and action. Kinematics of the right hand were measured while participants performed grasping actions or manual estimations under the guidance of haptic information from the left hand, binocular visual information, or both haptics and vision. The greatest uncertainty was observed with haptic information alone. Moreover, when visual and haptic sizes were congruent, performance was no different from that with vision alone. Although this gives the appearance that vision dominates, when information from the two senses was incongruent, an influence of haptic cues emerged for both tasks. Our paradigm also allowed us to demonstrate that haptic sensitivity, like visual sensitivity, scales with object size for manual estimation (consistent with Weber's law) but not for grasping. In sum, although haptics represents a less certain source of information, haptic processing follows similar principles to vision and its contribution to perception and action becomes evident only when cross-modal information is incongruent.