225/45/17 's give almost exactly the same total wheel size as the standard 195/65/15's that my '99 1.8 Mk4 GTI came with. (636mm total diameter).
I know this because I am considering some 225/45/17's myself, and I worked it out. I've not changed the wheels on any of my cars before, and I was wondering if I'll need any kind of spacers or something to let my arches/brakes accomodate the bigger wheels?