Due to anatomical differences, men and women (and children) must wear different chain lengths to obtain the same effect for appearance. For example, an 18" chain that hangs loosely over a lady's sternum may very well be a choker-style chain on a male.
Here's a picture displaying chains of varying length being worn. For reference, the person wearing these chains (me!) is 5'8" and a bit on the bulkier side (185lbs.)
From order of shortest to longest, the chains being worn 18", 24", and 32".
The picture can also be used as a reference when deciding how thick to get your chain.
the 18" chain is 2.5mm
the 24" chain is 5mm
the 32" chain is 6mm
The base of the 32" chain hits the bottom of the pectoral muscles, while the 24" chain hits the area right above. The 18" chain sits above the collarbones; there's approximately an inch-plus of clearance remaining on the 18" chain even though I have a rather wide neck. I'd imagine that most ladies would need to get their chains 3-4" shorter than the ones pictured in order to get the same look.
Hope this guide was helpful!