I have lots of different Phormiums and most of them get too big eventually! We have found that they can be cut back to about 12-18" with a hedge-trimmer or even chainsaw! It looks awful for a while but in a few weeks the new shoots spring up and it takes them quite a few years to get back to their former size. This is best done in late spring - as they are a 'grass'.

If you mean height, the above should work, if you mean the spread (width) then you need to dig out either the whole plant and split it into sections, depending on how large the rootball is, or just hack out some by the roots.