I have 18 Plumerias in 11 containers in a confined area. They range from 4 feet to 14 feet tall. And 4 to 20 years old. I live in Southern California. The plants stay outside and they're too big to move. I prune them regularly, but it's time for more than a pruning. How far can I go without damaging my plants?

@GigiAdeniumPlumeria

you can go as far as you want.
If you just want to prune off selected branches, you can do that. If you want to top the entire4 thing and cut the main trunk, you can do that too. Wherever you cut, the plant will callus over and branch and start over. I've done both methods before. Just cut branches, or cut whole truks, on plants that are 20 years old

Sorry I just read this with Gina. Mature plumerias respond well with pruning. I do it often when weatherman says we are going to have a chance of frost (mid 30s) as an insurance if my main trunk will not survive. You just have to protect the top end part of the plumeria from rain or water until it heals or callous. It is better to cut it in an angle so water will not pool on the top end cut. If you cut it flat, the end cut sometimes develops a "crater" as it starts to heal that can be the start of rot if it continuously gets wet. Wet and cold is not good combination even for mature plants.

Most of my plants have some leaves year-round but normally have no flowers, January, February and March how do I decide the best time to do this overall size reduction. .

That I'm not sure of. If I'm pruning, I do it in Spring or summer. They seem to make new branches very quickly then...this is the time of year they are trying to rest and be dormant. SO it would most likely be ok, but you most likely won;t see any new growth right now