The best, and neatest way, in my opinion, to make a fast chain for a pendant is to use the Foundation Double Crochet Stitch (fdc). I linked the page I used as a reference in the image below. In the pictures, a 2.75mm hook was used, if you want your stitches to be bigger/smaller adjust your hook accordingly.
Personally, I would not use larger than a 4.00mm hook for jewelry purposes. Just make it as long as you’d like! You can also begin the chain with a clasp end and then slip stitch the other end on when it is the length you desire.
Although, it was tough getting the yarn or string to not stretch out or for the finishing part to stay in place. I found regular yarn to be too stretchy and unversatile. I didn’t try embroidery floss, but this may work as well since it has a similar, more rope-like foundation to it than yarn does. Yarn was too “fuzzy” for this type of project (but works well for the clasped chains).
Thankfully I had this interesting string from some macrame projects from long ago. It is composed of 2 rope-like strings with a metallic string intertwined. It was a very taught string.
I knotted it around the pendant hole a couple times, then started my fdc with the starting string and the working yarn. I made it just long enough to fit over my head comfortably without a clasp. The next part was the most difficult: finishing.
I slip stitched the chain to the space between the pendant and the first chain 3 of the fdc. Then I did a double crochet (dc) in the first fdc space. Then I made a slip stitch to finish it off.
Instead of weaving in the ends, I crocheted the ends down towards the pendant hole, and knotted it again a couple times. Finally weaving in the ends and burning off the excess ends.