The only thing stopping me moving up to 18s on my '11 CITY is its arches
On a more serious note, 18s might be pushing it but i know a lad that has deep dish 18s with thin tyres on his pre-14 (reborn) Civic, yes that's in Faisalabad! All he does is make sure to drive slowly. Civic's suspension compared to CITY is so so better! You'd be less worried about cabin noises appearing or a harder ride than i feel! But ride quality will be very bad. All you'll hear us 'tuk tuk' over even shallow pot-holes, and you'll feel them too as tho you were coming down in a 911 GT3 on 20s!
All that said, it won't be all that bad if you drive slow on certain bad roads. The amazing look of 18s would by far offset the tradeoff on ride quality (it would for me anyway) versus aesthetics. It seriously won't be that bad.
Tyre size to go for - 235/40 IF it doesn't rub inside the arches (i think it probably will) or 225/40. 235/40 would be more forgiving altho won't be a huge difference. (I run 215/45R17, Dunlop LeMans LM704 with active foam technology - amazing tyres very very grippy but expensive)
Good luck anyway!