I have a friend with a very old plumeria tree in his yard.It is about 10-15ft tall.Is it ok to take a large cutting off it now?We live in central Florida.
Since no one else has chimed in, I'll throw in my humble opinion:
Yes, you can take a cutting now. It can be stored for up to 6 months in a cool, dry place before trying to root.
If you have a good warm spot in your home you could try rooting it over winter, too, though it will be much slower to root than if you wait. But, over-winter rooting will give the plant a head start for spring, too.
If you decide to try to root now, make sure the cutting has several days to dry and callus over the cut end first. Also, check out old posts for more detailed instructions on potting up and heat mats for your best chance of success.
And, btw, larger cuttings (over a foot or so) have a better chance of rooting than smaller ones, just make sure they're not too green (more mature grey/brown wood is better).