You can use 16", or 17" rims and tyres which are homologated for the Mercedes R 170 SLK 320.
Please take a look at the .pdf file from Mercedes Benz.
http://www.mercedes-benz.de/content/...170_100120.pdf
7J x 16 ET 37, or 34 with 205/55 R16
or
7,5 J x 17 ET 36, or 37 with 225/45 R17
There are also other combinations, but they are not homologated for winter chains.
So take a look in the pdf.
Regards
Markus