1 - If you are able to measure the chain of one of her frequently worn necklaces from one chain end to the other, that will give you a good indication of which length to order. All milestone necklaces are adjustable by 2 inches, so if her current favorite necklace measures 18", you will probably want to go with 18"-20"(adjustable) to give her versatility.

2 - Think about how the recipient will wear the necklace. Does she typically layer necklaces with pendants over clothing, or wear them at or near the neckline? What size pendants or charms do you see her wear more frequently? The Bold designs are the best option for long necklaces & layering over clothing. They are also beautiful at the neckline as more of a statement necklace. 

3 - Consider stature as much as height. Many of us assume that if someone is short, we should order the smallest size, or if they are tall, we should order the longest. Again, think about the preferences in #1.

4 - If you have limited access to any of this information, I always recommend the Mid Size options at the 18"-20" length. If the recipient doesn't typically wear jewelry, I recommend the Minimalists as they are light & easy to wear casually.