Users under 5 foot 4 inches should use a stride length of about 16 to 18 inches, while those who measure taller (up to 6 foot 7 inches) should use a stride of 18 to 21 inches. Proper stride length on an elliptical is determined by the user's height.
Without having enough space to fully extend their legs, tall users using short strides will be cramped. Likewise, short elliptical users using long strides will feel stretched and off-balance.
The best way to figure out what stride length to use is to test out several on an elliptical machine and see what feels comfortable.