To help give you an idea of chain length...
I can usually make a custom length chain, but these are the standard lengths I use.
14" (35.5cm) is a choker length and is also just the right length for children.
16" (40.5cm) falls around the collar bone and is a good choice if you like a fairly short necklace.
18" (46cm) also known as 'The Princess' is the most popular length of chain and falls just below the collar bone. If you're buying a necklace as a gift and are unsure of the right length to order then I would go for an 18", it can always be changed if you find it's not quite right.
20" (51cm), 22" (56cm), 24" (61cm) and 26" (66cm) are all known as 'Matinee' lengths and are popular if you like layering.
28" (71cm) and 30" (76cm) are 'Opera' lengths, as a single strand they lie about halfway towards the waist.
If you like the layered look then I find 16", 20" and 24" chains work well together.
You can always buy extra chains so you can have different length options for the same pendant.
I hope that helps, but let me know if you have any more questions!