This is a Poseidon pendant that was originally designed for a loop, or some form of clasp to fit in the hole and then be put on a leather cord necklace. Of course, that's completely optional and you can do as you like!

Sizing: It's 29mm around, and 29mm in height. You might want to resize this in your slicer to 40mm, which is a very good size for this pendant in real-life (which is about 1.5 inches). The hole for threading things through is 3mm, but once resized to the 40mm, that'll open up a bit.