Being boring and pratcial I suppose I personally don't like anything bigger than 17. Bigger wheels do look the business but can be a pain to live with.
Had 18s on the Cupra and they looked great, but was forever hitting them on kerbs when parking, go too close to a kerb and there the wheel takes the hit rather than the tyre.
A mate has 19s on his Civic Type R and he managed to clunk them most people do.
I got a refurb done on the Cupra as I hit one wheel really badly, the company who did it now sends me letter offering an annual subscription service for upto 6 refurbs in a year, seems to be a common
problem with bigger wheels.
If you park on the drive, do very little street parking you can probably get away with bigger wheels, otherwise the bigger the wheel the more risk of damage.
Its gutting when you've spent probably an extra £1000 or so once you've bought the wheels and tyres and then you go an kerb one a few weeks later.
Also remember to tell your insurance once you do the upgrade...