By far, the best option is to actually measure the recipient’s foot. Just remember that socks should have negative ease. Don’t make a sock exactly as long and wide as the foot. If you do, the sock will feel too big for the wearer. You need to make it a bit smaller.
Second Best Option
If they have a sock that they like and you can access it – you should! This is a great way to go because you’re already being told that those measurements – and that sock – work!
Just be sure to use a similar yarn and gauge so it has the same properties (including stretch and thickness).
If You Can’t Get Access to the Foot
There are times when you won’t be able to measure the actual foot. Either it’s a gift or the recipient just can’t measure (or you don’t trust the measurements). In those cases, standard measurements (and your own intuition) are the way to go.
Here are some measurements. You should just use them as a guide. The nice thing is that socks will stretch, and they should have negative ease as well. So, you’ll likely be OK if it’s “close enough”. Keep in mind that most socks are sold in just a couple size ranges. This is because they stretch to fit several sizes of a foot.