I think the length is great on you, and the pendant is stunning! Love the octagonal look, and the details on the back are incredible as well. Congrats on such a great piece!
I like the chain as is too but I think I would personally prefer a longer chain considering the size of the diamond. That being said, I prefer longer chains generally so I think it depends a lot on personal preference...
I generally wear pendants with a 17 inch chain, but when you want to wear it out over a sweater or higher neckline, for example, you can purchase a necklace extender of 2 or more inches to make the necklace longer without having to have a second chain.