This study presents the geometrical optimization of a solar air heater. A reasonable working range of geometrical parameters: length 1–3 m, width 0.1–0.5 m, and height 0.005–0.05 m (or aspect ratio 1–20), is investigated as per general applications and installation constraints. The effect of these geometrical parameters on thermal (heat flux and outlet air temperature) and hydraulic (air velocity, mass flowrate, pressure loss and pumping power, and Bejan number) parameters are theoretically studied. Later, a numerical investigation is carried out with an optimum geometry of SAH-duct (length 2.44 mm and width 0.3 m), for aspect ratio 4–18 and Re 3000–15,000. An experimental study is also done to validate numerical results. The heat transfer and frictional loss do not show a significant change in the values for the aspect ratio 8–12.