SunnySports S Using a flexible tape measure, measure the length of your torso from the C7 vertebra (at the base of your neck) to the spot on the spine level with the top of the bony hip structure called the iliac crest. Do it two or three times to get an accurate measurement. If you are between frame sizes, always go with the smaller size. If the torso fits, the pack will fit. Stay safe and have fun out there! Neal