First take a piece of string then make it into a sling. Experiment. I still don't know how long a sling should be, how large the pouch should be or what it should be made from and I've been slinging for something like 4 or 5 years. I have slings made from paracord, jute, cotton string, yak hair, other types of synthetic material with pouches made from leather, braided, split-pouch, even a 3D printed pouch. If it can be made into a sling, chances are someone on the forum has tried it. I've even seen plastic shopping bags adapted into slings.
One measurement I've seen, and I would suggest for a beginner is measuring from your outstretched hand to the center of your chest. This will be the size of the sling when it's folded in half. Having said that, I have slings of all different lengths and thicknesses and weights and, as I said, materials. Experiment. See what works for you. All of this also goes for ammo