A theoretical study on the maximum evaporation rate obtainable in a small-scale indirect solar dryer is presented, considering evaporation of free water. A mathematical model of the evolution of the temperature and the specific humidity of the airflow along the drying chamber is presented. Based on the results, some simplifications are proposed and justified in order to calculate the maximum evaporation rate as a function of a reduced number of parameters, to study their effect. The results show that the effect of the air mass flow rate on the maximum evaporation rate depends on the aspect ratio of the drying chamber, defined as the ratio of the total drying area to the cross section in the drying chamber. Design and operation criteria can be extracted from the results. As a global result, for the typical range of dimensions and air mass flow rates employed in solar dryers, the drying chamber aspect ratio should be between 200 and 300 to obtain a proper evaporation rate. Within that range, doubling the air mass flow rate the maximum evaporation rate obtainable increases around 20%.