^^That was some excellent information about tires...
But I think what most people want to know is what are the best sizes for the 3rd Gen Altima specifically. It starts with the rim that you have our going to be getting.
If your upgrading to a bigger rim like an 18" or 19" or even a 20"....
an 8.5" wide rim seems to be the widest with a +50mm offset at least.
8" wide rim is really the best width with a +45mm offset, you can get away with +40mm with a 8" wide rim. Anything between +40 to +45mm offset is what you want. Anything smaller then +40mm you need a 7 to 7.5" rim or you will have rubbing issues. You have a 8" wide rim with a +35mm offset you might get away with a 225, but thats a narrow tire for the rim size. A 235 may even still have a rub issue.
Tire size, 245 are about the widest for best fit on a 8" or 8.5" wide rim. Some have tried 255, but not without rubbing issues.
The idea is to keep your new wheels & tire selection close to stock diameter..
use the tire calculator blitzboi gave is really helps, I use this one:
http://www.powerdog.com/tiresize.cgi
Note: are lug pattern is 5X114.3 or 5X4.5" the lugnuts are M12X1.25 RH