what size do you suggest for my 04 black mustang?

 

If you want a rough, sporty ride go with a bigger rim (the bigger the rim the more likely it'll get damaged though). If you want a smooth ride you want a smaller rim (with bigger tires - bigger sidewall means better road absorbtion).

 

i've had 18's for about a year now and i havent had a problem yet

 

A year is not a long time and I'm assuming you haven't hit any considerable potholes or debris. If you hit something you can bet if you don't have enough tire sidewall your car is absorbing the impact instead of your tire.