255 is the widest tire you can put on a rim physically. However, during cornering at even 5/10ths of the potential of the entire car will almost guarantee you rolling the bead and probably ripping the tire off the rim. Keep your shopping to the 225 (or lower) section of the rack. IF you do somehow get 275s on a 7 in. rim (which would be almost impossible - especially on a Civic) your performance will suffer. More tire is not always better.

I have no idea what profile you are looking at so I cannot answer in such a way to help you on putting as much tire to the ground as possible.

You see the profile AKA aspect ratio is totally dependant on the actual measurement of the section width. So with a 225/45 the 45 means the tire sidewall is 45 percent as tall as the tire is wide. The 225 is how wide the contact patch is and is measured in millimeters - 225mm.