okay.. my car is lowered 30mm and this is what i have found..
14" is the biggest size i have found to fit on a cinq without any work (I had 6x14" with 185/50 tyres)
15" fitted with no work at front but i needed my arches rolled at the back to stop them catching. I have known people who say 15s will fit with no work if the car is not lowered but i wouldnt know that for certain. Mine still scrubbed occassionally with a load in the boot / back. These were 6.5x15 with 195/45 tyres.
17" can be fitted, as can 18" apparently. I have seventeens now, they were fitted by Xtreme in Harlow, who also sorted out the arch work. This did, in part, have to be re-sorted by my bodyshop as they had crinkled and dented some of the bodywork over the wheels.
Hope that helps.. if u want to see what each size looks like, theres pics of my car with 14s,15s and 17s on my web.
www.btinternet.com/~flippedcinq
fABULOUS, fEMALE aND tOTALLY fLIPPED