How do you size the correct diodes for OR application of two power supplies?
Each diode needs to be specified to withstand the highest current thru it and the maximum reverse voltage across it.  This really should be obvious.

Keep in mind that the limiting factor for current may be power dissipation, not the maximum current spec of the diode.  You have to check both separately.  For example, let's say the maximum current you decide the diode will ever have to conduct in steady state is 5 A.  You check the datasheet and find that the forward drop at 5 A is 950 mV.  That means the diode will dissipate (5 A)(950 mV) = 4.75 W.

Now you look in the datasheet and see if the diode can safely dissipate 4.75 W in free air at your worst case ambient temperature.  If not, you need a different diode, a heat sink, or some form of cooling.