It will be a chore to get the chunck to finish in that micro space. if you transplant it, it will still get some excellerated growth after it takes hold after about a week. So I would think you will have about two weeks after that to move it into a bigger space. Even if you go 12/12 right away. You can try to lst but I don't see it staying under 18 inches. You now have this experience to rely on, so as long as you learned something, it will still be a positive.
Peace
Eek