How do you choose a diode for a 5v to 3.3v clamping circuit
Use a diode that is fast enough to handle the voltage changes on the line.
Check for the current that will flow though the diode and make sure it can handle that.

Simply, a Schottky diode with a rating of 50mA to 100mA should suffice.

Alternatively, use a voltage divider with a division ratio of:
5V/3.3V = 1.51

It will also work for 3.3V:
3.3V/1.51 = 2.18V

Because 2.18V is above the minimum voltage level for logic 1.