This paper presents results of an experimental investigation of a relativistic L-band (1.3 GHz) 10-cavity rising-sun magnetron with cathodes of various diameters. A major goal was to investigate the effect of the cathode diameter on the overall performance, mode stability and pulse shortening of this magnetron when operating at voltages of 100-500 kV and microwave peak power of P-mu similar to 100-700 MW. The largest levels of Pmu similar to 700 MW have been achieved, as expected, with cylindrical velvet cathodes. Mode structure and mode competition/switching for each cathode has been investigated in the entire range of operating magnetic fields., A cathode plasma expansion was found to be not the primary factor in a mode instability and pulse shortening of the magnetron investigated. Experimental results on effective magnetron diode gap, uniformity of electron emission, variation of the peak microwave power and microwave pulse duration with the cathode diameter and a correlation between the magnetron operating point and Buneman-Hartree curves are also presented and discussed.