A stepper motor or step motor or stepping motor is a brushless DC electric motor that divides a full rotation into a number of equal steps. The motor's position can then be commanded to move and hold at one of these steps without any position sensor for feedback (an open-loop controller), as long as the motor is carefully sized to the application in respect to torque and speed.
Switched reluctance motors are very large stepping motors with a reduced pole count, and generally are closed-loop commutated.
Changing the stack length will generally not impact on the speeds that you can get but it will have a major impact on the torque (turning force) that you are able to achieve.
For example the ZD2N2318 and ZD10N2318 stepper motors are both NEMA 23 motors (therefore 57mm diameter) but the ZD2N2318 is 42mm long whereas the ZD10N2318 is 104 mm long.
The difference in torque between the 2 motors is 0.6Nm for the ZD2N2318 and 2.4Nm for the 17hs19-2004s. The difference in stack length of a motor with the same NEMA rating has therefore quadrupled the possible torque.