I have a 2003 Audi A4 convertible with standard 16inch alloys. 

 

How big can I go without spoiling the ride/handling? I don't really want to change anything else apart from the wheels though, so not bothered about too large. 

 

I was thinking maybe 17/18inch?

 

Any help would be appreciated from anyone that has gone bigger. 

If you went to 17 inch it wouldn't make too much difference but 18 inch you would probably notice it. Increasing wheel size will always make the ride harder and the tyres will become lower profile.