It's all really subjective and for me it also has to do with shape, size etc. and everyone is different!
For example, to get the look that say
canyongirl (I really hope you don't mind using you as an example!

) has with her lovely 18" chain and key pendant, I just know straight off I would have to use a 20" chain to have the same effect.
Imho, the Tiffany chains of 16" are too short to be sold as a 'regular' length - it's very high on the neckline but that's what some people might want, but personally it's not for me. I recently bought a Tiffany pendant and went for a 20" chain which I'm very happy with.
A good dressmaking tape measure is your friend in these situations as are any necklaces you might own (and measure to get some sort of comparison).