It depends on your size man; everybody's a little bit different. It also depends on how low you want your necklace to rest on your chest.
I'd suggest taking a string and wrapping it like a necklace around your neck. Adjust it till your necklace item would hang at the point you'd like it to. Unless you know a professional or got a friend to help you out, you're probably going to have to figure out yourself how long the necklace needs to be to get your desired effect. Try doing it while wearing the clothes you'd like to pair the necklace with. That might help.
You may have to round to the closest size in inches if you're in-between.
Cheers,
Alex