Today there are dozens of sizes of drawstring bags available, as well as different cord lengths. This can make it difficult to find the correct size you'll need, and this goes double if you're making your own cinch bag.
Bag Size The first thing you'll have to do is estimate the size of the largest product you want to put into your drawstring backpack. As a general rule, you want to add at least 0.5 inches up to 2.0 inches larger in size than the largest items you want to carry. This will give you enough room to pack your things in without feeling constricted.
The same goes for the length of the drawstring bag. You want to have at least two inches of room to maneuver your things around. So you start by estimating or measuring the size of the largest objects and purchasing a bag that is two inches or more longer than your object. This allows you to have enough space to cinch the bag closed safely.
Drawstring Length Most drawstring lengths are standard, and it will take some maneuvering to shorten them. You can get a general idea of how long you want the drawstring to be measuring your torso length and comparing it to the drawstring length. To get your torso length, start by finding your C7 vertebra, or the bony spot where your shoulders and your neck meet. You can tilt your head forward to feel this area better. Next, slide your hands down until they're resting on the top of your hip bones. Point your thumb backward and draw a line across your back to the center of your spine. Have someone measure the distance between the C7 vertebra and the line that was formed across your hip bones. This is your torso length, and you can compare it to the drawstring's length to get the correct fit.