255 will fit on an 8.5" wide wheel but the sidewall will bulge a little. This is my old car (yes I know not a Civic) on its winter setup. Rears were 255/35-18 on 18x8.5. This was the stock size.
If you don't already own these 18x8.5s you can fit an 18x9 +38 or 18x9.5 +45 no problem.