Those are wonderful cordylines, but in the wrong place. It seems like vandalism to shorten them. Can't you move them somewhere else, where you might be able to appreciate them as they are? As previously noted, they don't have huge root balls, and can be moved, if you keep enough of the root ball.

You'll just arrive there again if you shorten them. If you like that Dr Seuss look, but not the height, why not grow something of roughly similar appearance, but with less height, such as a some hardy yuccas, or furcreaea, or astelia. Or, as lannerman suggests, one of the red Cordylines that are much more slow growing, in a tub or not. Though the red ones are more prone to cold damage.