For a maximum current of 50 mA the capacitor must be from 470 nF to 250 V if the grid is 110 V and 330 nF x 450 V if the grid is 220 V. The zener determines the output voltage and the diodes of the bridge can be 1N4004 or 1N4007. Remember that this power supply does not have any insulation from the power grid.
