If your Phormium is getting a bit too big for the space, as this one did, it's very easy to make them smaller.
All you have to do is dig up the whole clump, then shake them gently to get the soil off the roots, and you will find that they fall apart into a number of smaller plants. If it's a very old clump, as this one was, you might need to use a hand tool to gently lever the clumps apart - second picture.
You can then choose the best of the smaller sections to replant in the original position, and either put the other sections elsewhere in the garden, or pot them up for swapping or selling here!
I find it's best to cut the fan of leaves down to just a foot or so in height, otherwise they fall over and uproot themselves: they start to regrow almost immediately, from the centre. It does look a bit odd for a while, but as soon as you have a few new leaves showing, the shortened outer ones can be cut back even further so they don't spoil the effect.
If you wanted to keep the instant height, just push some garden canes in front and behind the fan, as closely as you can, to support it while it settles.