Hi Jeff,
Welcome to the forum! You'll find lots of very helpful, knowledgeable people here.
For choosing tire and wheel sizes, you have to determine a few things first...
1. Which model car do you have? Is it a Mk III or earlier?
2. You mention 5-bolt. Is the rearend a fox conversion, a stock rearend from a '94 or later?
3. Where you thinking of 15" wheels or another size?
4. I believe there is also a problem running wider front wheels (more than 8") if you use the stock mustang lower A-arms instead of the optional tubular A-arms.
Once you figure these out, the forum members will be able to be a lot more helpful.
Also, as a starting point, check out this page on the Factory Five site. The chart at the bottom will give you a good starting point for wheel sizes, even if you don't like that particular style of wheel.
http://www.factoryfive.com/table/whatsnew/wheels/bulletwheels.html
p.s., on the forum it's considered impolite to use all capital letters. IT'S YELLING.