Can you put 18 inch rims on a 2004 Mazda 6 without any modifications. If so what's the biggest you can go?

yes any front wheel drive 18" wheel with a 225/40r18 tire will work but you must roll the rear fender lip up so no rubbing will happen 19" is an option as well tires size 215/35r19 NEW ANSWER: 20'' wheels is the biggest you can go, with 225/30R20 tires.