What is a reasonable current rating for a catch diode for switching applications?
You have to look at the duty cycle of the converter, which in continuous conduction mode is ~ D = Vout/Vin.  Then the diode is carrying the output current for (1-D) of the time.  So take the max output load current times (1-D) (at worst case Vin) and add appropriate derating.  

Then check your thermals.

Diodes can typically handle peak currents well in excess of the average, so the peak current in a buck converter isn't usually a concern.