<p>Planar Josephson junctions in a magnetic field exhibit the superconducting diode effect, by which the critical current magnitude depends on the polarity of the transport supercurrent. A number of different mechanisms for the effect have been proposed. Here, we study symmetric, T-shaped planar Josephson junctions with semiconducting weak links in an in-plane magnetic field perpendicular to an applied current bias. In particular, we vary the longitudinal width (i.e. parallel to the current) of the superconducting contacts and the voltage of an electrostatic gate. We observe that both the critical current and the diode efficiency increase with contact width, and relate the critical current behavior to the induced coherence length of the Andreev bound states conveying supercurrent through the junction. We further observe a linear trend, with respect to inverse contact width, of the field at which the diode efficiency is maximized, which saturates as the contact width becomes large compared to the coherence length. The smaller field at which the critical current is maximized additionally exhibits a strong gate dependence. We interpret these observations in the context of multiple underlying mechanisms, including spin–orbit coupling and orbital effects.</p>