In my opinion only, I think 17in rims are about the biggest you can go on these cars without it looking "awkward"... I know many people are happy with their 18s, 19s and 20s on their cars, but to me the 17s are about the right look on these cars.
The 17s may even look funky on the W108/9 and compact W114/5 but they seem to be a popular choice for younger-aged oldtimer owners in Germany.
I made comment a few years ago about wanting these wheels but alas piggie bank is low at the moment and just fitted refurbished penta wheels. Good luck selling any way.