When designing or repairing an electronic circuit, you should be aware of diode characteristics, especially those that, if you exceed them, will destroy the part. Two of a diode's more critical characteristics are peak inverse voltage (PIV) and maximum forward current. PIV is the highest voltage a diode can tolerate in a reverse-bias direction. Maximum forward current is the most current the diode can conduct in the forward direction. Generally, larger PIV and currents mean physically larger, more expensive diodes. Sizing a diode means finding one that best suits the circuit for a reasonable price.
Examine the schematic and determine the most current the diode will have to conduct in its forward direction. Also determine the maximum voltage the diode will have across it in the reverse direction---positive voltage to cathode, negative voltage to anode. If the circuit uses DC only, you can size the diode by taking these voltage and current values and adding 20 to 50 percent for safety.
Look through the diode catalog for a part that can handle the voltage and current you determined in step 1. If the circuit uses pulsed current instead of DC, also look at the peak current forward current rating specification in the catalog. As with the other ratings, you'll want to allow at least a 20 percent safety margin.
Note the diode's packaging or case style. Larger diodes will dissipate more heat, so they'll be packaged with a metal heat-sink tab. Even larger diodes will look like metal bolts, meant to attach firmly to large conductors and heat sinks. If you're repairing a circuit, make sure that the new diode's case style matches the old diode's. If this is a new circuit, take the diode's physical mounting requirements into consideration as you plan the circuit board and case.