Measure to the spot you want them to land.
Height alone does not determine short length. Someone who is 6'2" will need a much longer length than someone who is 5'2", and even two people the same height can need different lengths depending on leg length, torso length, and where they wear their waistband.
For the best fit, measure either:
Inseam: from the crotch seam down the inside of the leg to your desired length.
Outseam: from the top of the waistband down the outside of the leg to your desired length.
