Motion generation or rigid body guidance for planar four-bar linkages is a classical problem in planar kinematics. This paper introduces a novel metric for motion approximation that guides a bounded planar object rather than an infinite plane through a set of given positions. The metric is based on an estimate of an object size using the average squared distance (ASD) among all homologous points of the bounded object. In this way, the motion approximation problem is reduced to that of finding the linkage dimensions that minimizes the sum of ASD distances for all given positions. Numerical examples are provided to compare the proposed metric with three other existing metrics for motion approximation without taking into account the size of the moving object. Also explored is the effect of the object size on the dimensions of the resulting four-bar linkages.