haha, alright...finally a question about a bronco! haha. anyway, fullsizebronco.com is a great site. I am a member there, and have learned everything that I know today from that site. Join up. There are alot of good guys and gals over there. But DO NOT just jump off asking a question like that. You will get ridiculed, and made fun of because there is a new member who asks that question just about every day. There is a whole sticky dedicated to tire size, and lift size...so use it. Use the search function wisely and introduce yourself in the introductions forum.
To answer your question, the biggest tires you can fit comfortably on your stock 89 bronco are 33X12.50's. That is with a bit of trimming of the front bumper. Sure, you can go bigger, but that would require ALOT of trimming, and maybe even some new fender flares. But I assume you have the 3.55's in your truck? You dont want to go any bigger then 33's without re-gearing anyway.
TOTALED DEC. 2014...05 XLT 4x4
Replaced with Blue Jean Metallic 2014 SuperCrew F150, 5.0 Coyote, 10% tint, Hell Bent Steel Leveling Kit, Allied Duals, Weathertecs, Uniden CB