I am glad you finally admit to the old and sad bit (j/k). To be honest I am not a fan of too big either, there need to be meat on the wheel. Too low a profile and it looks like someone took a roll of electric tape and made a pass .. .
But on this car all the rims came with. Personally I like the winter rims the best. Those are 16 inches and if I am not mistaken they are sporting 205/50 -16 (it might even be 55) . The 17 inch ones are 225/45-17 .
18s I think would be too big on a Xantia. I've seen a few , it looks modern but not quite my cup of tea.
As to what size I will end up with in tyres I am not sure. I visited this one page ;
http://www.superdekk.no/produktsok/dj67096 (my reg number - so the car in question is a Xantia. In the pull down menu its says produkttype, same as in english. you can choose Dekk (tyre), Felger (rims) or batteri (battery). Dekkstørrelser when choosing Dekk is Tyresize.
I think 225/55 - 16 would be a nice choice for a 40 year old youngling
