Most of the people on this forum use needles and not looms, so I'm not sure how much help you'll find here. One thing I will suggest is making some swatches with different numbers of pegs and using different yarns. Then make note of how you did these swatches so you can reference them later.
One thing to keep in mind is that socks are generally knit pretty tightly and are expected to stretch a bit to fit your foot, which is why sock yarn often contains nylon.
If you just want to jump in, you can do that, too. Socks aren't so huge that you agonize over ripping out several inches. And after all, if you make socks that are too tight for an adult, you can always adjust the foot length and give them to a child or even a baby.