This paper presents a reasonably accurate analytical method to size and to design a single rotor single stator Axial Flux Permanent Magnet machine under iron circuit saturation. This approach allows obtaining the electromagnetic torque for a given value of tangential stress and outer diameter for the optimal diameter ratio, but can be used for any other diameter ratio. Analytical approach, avoiding numerical solutions, allows one to achieve the main dimensions of the machine in a much faster manner. By the introduction of a stator leakage flux factor and a saturation torque factor into the model a more precise estimation of the torque capability is obtained. The results show that the leakage flux and the iron saturations significantly reduce the torque developed especially, if high levels of tangential stress are reached. Finite element analysis was carried out to validate the proposed method showing acceptable results on the analytical predictions.