Does your doll have an older style O-ring or D-ring at the top of the neck instead of an S-hook? Are you a compulsive restringer? Do you enjoy replacing your doll's torso or leg parts, or would you rather have an S-hook at the top?
Well, my advice is to avoid the hardware store, and buy hooks directly from a BJD dealer. You pay a little more, but will get hooks that will last a lot longer and actually be the right size for your doll.
Several places have the right sizes: