To the OP, they are not talking about the length of the stem but the height where it clamps to the steer tube. Your new stem is probably a little smaller so it is not going to work without a spacer. To give you a visual, look at this photos taken from Slowtwich. The steertube must be 2-3 mm lower than your top cap as Hillrider and techsensei have advised. The spacer above or below is up to you as long as you have a spacer.