As far as sizes go, there is excellent data on The Tire Rack's site:
http://www.tirerack.com It takes a little chasing around to find eveything, but when you click "specs" for a tire, you get a list of available sizes. They list all dimensions, including revs per mile, so you can match diameters to keep the speedo in calibration, and width, so you can decide what "might" fit without too much hassle. Also, required rim widths, so you don't buy a 265/30 to pop on your 7" rim. (Speaking of which, when I checked they had 56 wheels for Vibe!)