how big of tires can you go on a civic ex coupe?

 

http://www.dakota-truck.net/TIRECALC/tirecalc.html

you can go there and use the conversions in the middle of the page to convert your current size to an equivelent larger size and it will also give you speedo error margins.

i know most 'ricers' use 205/40/17 it offers the least speedo error. (for a 185/65/14 wheel that is)

 

I think 215 is the max without sticking out pass the fender and looking funny.