My take is that it depends on what generation of Civic you have. 18s are a bit too big on 6th gens and older. 7th gens are pushing it. 8th gens can easily get away with 18s and daily driving.

 

It depends on how much rubber you can run. Ideally a 45 or above is going to create the best ride quality.

 

My 17's were uncomfortable, but still liveable IMO. And since you're asking about the largest wheels, I assume ride quality isn't an issue, so I say 17's or 18's, depending on generation... personally, I'll never go more than 3" bigger than stock.

 

go with 30" DUBS, they'll fit just fine.