How big of a tire can i put on my 2000 wrangler without a lift kit, its a 15 inc

31's should fit fine and not be TOO obnoxious. Anything bigger than that and you might have to start worrying about fender flare rubbing or rubbing your components and need a wheel spacer. With 35's, you'd need to start looking at a 3" lift at least and starting to trim some metal. with 38's, you have to upgrade your brakes (which you can't do with Jeep axles) and go to about a 6" lift and frankenstein your fenders drastically....and upgrade your steering stabilizer piston...change your pitman arm on your steering and too much other suspension stuff. 31's will make it look trail-ready without needing any other modifications.