If you're referring to a diode connected "backwards" across each winding of a stepper motor to protect the driver IC from back EMF from the coil, the important characteristics are:
1. Maximum reverse voltage. When the winding is energised by the driver IC, the diode is reverse biased. The full supply voltage appears across it in the reverse direction. The diode's maximum reverse voltage rating needs to be at least equal to the supply voltage, and preferably 50~100% higher than it.
2. Continuous forward current. When the driver switches off and the magnetic field in the stepper motor coil starts to collapse, the "flywheel" effect causes current to flow through the diode. This current is initially roughly equal to the current that was flowing from the driver into the motor winding. The maximum value of this current can be calculated using Ohm's Law, from the supply voltage and the DC resistance of the winding. For example if the supply voltage is 30V and the DC resistance of each winding is 10 ohms, the maximum current will be 3A. The forward current rating of the diode needs to be at least this high, and preferably 50~100% higher.
3. Switching speed is important, especially if the motor is running fast and/or efficiency is important. This is why I would not use a 1N400x diode. A fast recovery rectifier or a Schottky diode would be a better choice.
This diode is typical for this kind of application. It's a Schottky diode, rated for 5A and 50V.
http://www.digikey.com/product-detail/en/SB550/SB550FSCT-ND/3907898