This paper revisits the classical problem of approximate motion synthesis of four-bar linkages for rigid-body guidance. Instead of treating it as a planar pose guidance problem, this paper focuses on the guidance of a portion of the moving plane with finite size. The paper introduces a two-stage algorithm for optimal approximation of planar four-bar motions using an object-size dependent approximation error. A quasi-linear formulation of motion approximation is adopted that allows for rapid generation of initial sets of dyad parameters from all given poses of a planar object. The resulting dyads are then combined to form four-bar linkages and subsequently fine-tuned by minimizing the object-size dependent approximation error subject to both dyad constraints of a four-bar linkage. A numerical example is provided to illustrate this novel approach.