a stepper motor rotates the shaft in steps or 1/2 steps. there are upto about 1600 steps per revolution depending on the motor. the step size is govened by the windings of which there are usually 3 or 4. the windings are aranged like so:
12341234123412341234
the windings are excited in order and the shaft will move to the nearest excited winding this generates the motion.
By size i take it u mean torque rating. If so i would like to point out that speed and torque and inversely proportional. by this i mean that as the motor speed increases the less load is required to stall the motor. stepper motors are usually only used in applications where either constant speed or accurate positioning with out feedback is needed.
Hope this is of some help post again if you need to know more. I have tried to keep this explanation simple as i don't know how technical you are.