I have 16'' rims in X-Type and 14'' in Rover 416 because older roads around my city tend to be still pretty rough.
Larger wheels would probably look better, but comfort would suffer as well. Not to mention that "smaller" tires are much cheaper.
My bro has Vectra on 17'' wheels (due to 3.2 engine) and on rough road it is unacceptable for me.
On smaller rims at least I don't have to look jealously at my other brother's C5 with Hydractive 3+ suspension or wonder how (too) much X350 would cost