This is best done with someone to assist you. Tilt your head forward and feel for the bony bump where the slope of your shoulders meets your neck. This is your 7th cervical (or C7) vertebra, and the top of your torso length. Now place your hands on the top of your hip bones (aka the iliac crest). Point your thumbs at each other across your back. That imaginary line is the bottom of your torso. Have someone use a cloth tape measuring tape and measure the distance between your C7 and that imaginary line that your thumbs are making. That is your torso length. It does not have to be exact so don’t worry if your thumbs aren’t parallel to the ground when they are pointing at each other, or where the exact center of your C7 is, etc.