The nuclear and magnetic structures of eight compounds belonging to the Pr0.5Sr0.5-xCaxMnO3 system have been studied, by means of neutron and electron diffraction, in addition to electric and magnetic measurements, to understand the role of the average size of the A-site cations, [r(A)], upon the physical and structural properties of the series. A structural and magnetic diagram, in the ([r(A)] or x, T) plane, has been established that shows the complexity of the behaviors and evidences the relationships between magnetic and structural transitions.