Since I don' thave to worry about brake clearance, I picked up a used 15" steel wheel off a mid '70s Coronet from the salvage yard for $10. Put a used 235/60 I had lying around so I have a similar diameter spare.
I wouldn't recommend going much bigger than 235 in width because once you get it up on the riser in the trunk and try to stuff it up under the package tray, you run out of room pretty quickly. Even at an angle, a 255 could be a challenge to get in there, but each car is a bit different.
If you are running larger than 11.75" brake rotors, you will need to look at oversized rims for spares.