I have black victory spoke wheels on my midnight black Cooper and I am happy with the way they look, The overall size of the tyre is the same as a larger wheel. I think it would be more noticeable if the wheels are not black.
Have to agree that whatever size you have the outside diameter is the same, the plastic wheel arches really show up the smaller alloys. Mine are 15's and they look very small but other sizes look that way as well in my opinion, the black disguise it better. I had a Scirocco which I put 17 ins winter wheels and alloys on, they looked very small in the wheel arch compared to the normal 18's even though they where same diameter.