Depending on the stepper motor inductance, there will be 1/2 LI^2 Joules to dissipate. Judge the impedance of the circuit at switch off. Now calculate the PEAK current in your diode. Yes its a very big number....

For a < 1 A stepper, you can often JUST get away with a 1N4007. You must use a power diode, and not a signal diode.

Steve