Well I just pulled out two very large red Phormiums last weekend. They had been in the ground for 5 years, were about 24" in diameter and 6' tall. They had been cut to the ground twice and trimmed back several times each year. After awhile they need to be dug and divided, unless you have plenty of room for them to just keep growing.
I cut them back to about 18", dug up the whole thing and then pulled each of the clumps into about 20 seperate plants. Each of the pieces is about what you would get in a 5 gal nursery purchase. They have been banished to the back hill and will probably take a year or so to get going again.
If you just want to prune them, I would start by removing any of the dried fronds/leaves that are in the clump. Then I usually start at the outside bottom and keep cutting fronds until the plant is the shape and size desired. They can become unsightly unless you prune the outside pieces down to the ground. This usually promotes a lot of new growth.....which is sometimes not a good thing. :-) I pruned one like this in the front last fall and it has doubled in size.
If the plant just has bent, tattered, torn leaves, just reach down as far as you can and clip those off with a pair of pruners. If the remains of the leaves are left on the plant, it can promote a thriving colony of mealy bugs. They love this stuff.
