I have a 90 f150 4x4 and i was wondering how big of tires i could fit with a 8in suspenion and 3in body lift

 

You should be able to fit 40s with that amount of lift, and still allow for some room to flex. Have fun rebuilding the front axle every other week with that size tire and with an 8" lift. Beyond 6" suspension lifts the TTB becomes flimsy at best.