If your anklet is made of beads, larger beads will affect the optimum length because of the drape factor.
Chunky beads will mean a longer length.
Tiny beads will mean a shorter length.
How to Measure For Anklet Sizes
Sizing For Small-Bead Anklets or Chain Anklets
To measure for a chain anklet or an ankle bracelet with small beads, here's what you do:
Take a piece of non-stretchy string and encircle your ankle snugly (but not tightly). Measure the string, then add about a 1/2 inch. This will give you a pretty good size estimate for an anklet that falls below the ankle bone.
(I like to use a length of chain or string, then measure the string afterwards because measuring tape somehow just doesn't sit right.)
Sizing for Larger-Bead Anklets
If you are using larger beads, you will need to add some extra length to compensate for the extra bulk of the beads.
The easiest way to make sure you size correctly is to measure as for a chain anklet then test the sizing after you have strung your beads, but BEFORE you finish off the ends by test measuring around your ankle.
If you are making the anklet for someone else:
To test measure for someone else, try to get a measurement surreptitiously (like when they are asleep) and then use the measurement to make a tube out of cardstock. Pretend that's your ankle and drape the unfinished ankle bracelet around your pretend ankle.
Making an Anklet Adjustable
I always find getting the length of an anklet or bracelet tricky. If you do too, you might want to use a necklace extender or chain so you can adjust the size.
Adding a necklace extender to a piece of jewelry is pretty easy. Just be sure to use something secure to attach it to your anklet. Using a single jump ring is not a good idea because the jump ring can easily pull open if your anklet gets snagged on something. (Necklaces don't get as much abuse so often a jump ring is all you need). Instead of a jump ring, you should use split rings or make a wrapped loop.