well its depends what do u wanna do with ur ride,
r u gonna use it for daily purpose or occasionally, if u use it for daily purpose then i wont recommend 18", i would suggest 17" max
also depends what do u want from ur ride, (performance, outlook or ride/suspension)
if u put anything 18", then ull loose performance and u might loose the handling as well
18" defintely will look good on ur car, but also ur ride will be too bumpy, ull feel every single bump on the road, and ur tires might touch ur quater panel if u go hard on bumps
i wouldnt recommend 18" for civic cus it got small engine, if u put on v6 accord then its good
i got 19" on current ride rite now and its ok cus my car can handle it i can put up to 22".