Cutting back by half should work well. Before Heirloom got rid of their gallicas they used to cut them back by two thirds every three years or so.

Shape the plant with hedge shears after its finished flowering. If you need to keep it under 5', then you will have to repeat this pruning once more, in all likelihood, in mid/late-Summer. Once established, this can be a rather large shrub and you will have to work to keep it at 4-5 feet.

Regards,
Paul

Thanks to all. I'm thinking of adding some supports so that I can allow it to get larger. Or, move those pesky other plants that it's encroaching on.