I stumbled on this necklace sizing chart and I think it's a useful tool to help someone who is unsure about what length they need, especially if they are looking for a gift. You can get a pretty good idea from this model, who I think represents about average. I have a "thicker" neck and shoulder structure, so a 20" hangs closer to the charts 18" mark on me, or in between.
For pendant necklaces, consider the size of the pendant itself as well. I measure the strand(s) of my pieces and the pendant separately.
It is always possible to lengthen a necklace with extension chain (unless the clasp is magnetic-then it has to be restrung), so I started making shorter pieces. It's easier to do it that way than trying to shorten a piece that is too long.
When shopping for bracelets, consider the size of beads and components. Larger beads will require more length equal to the bead size for a proper fit.