What do we mean by "drop"?
We use this term both with dangly earrings (as opposed to studs), and pendants.
In the case of an earring we mean the length from the top of the loop (that goes though your piercing) to the lowest point of the earring.
In the case of a pendant we mean the length from where it is suspended on its chain, thong, ribbon, etc.
"Length" would seem to be an alternative term however in some cases there might be a projection above the point of suspension such that the length was greater than the drop.
Pendant Chains & Thongs
Most pendants are supplied with an 18" chain or ribbon thong (as stated in the description). Sometimes there are options.
The best length for you will depend on factors such as the shape and size of your neck and shoulders, and where on your chest you want the pendant to sit. The best way to determine this is by measuring a piece that you already own.
Bear in mind that the weight of a pendant will pull a chain into more of a V shape than you get with an empty chain.
BraceletsBracelet lengths are stated as the length of the chain and NOT the wrist size for which they are intended.
By far the best way to know what is right for you is to measure an item that you already own.
Failing that, the next best option is to wrap a thin (no more than 0.5") strip of paper around your wrist, open it up by about an inch, fix it with tape, and see how it fits.
A bracelet should have some slack... but not so much that it slides right over your hand and falls off. How much slack is a matter of taste. Some people like a bracelet to stay close to their wrist while others like it to slide some way onto their hand.