Sorry if this doesn’t necessarily belong in exterior, but I want bigger tires on my 1996 F-150 XLT Regular Cab 4x4 but I’m not experienced with it. How big can I go before I encounter rubbing? How big could I go with a leveling kit? How big could I go if I decide to but a 4” suspension lift?

 

33x12.5 is (was) a common add-on size back in the day.

Will be crowded on stock height, not too bad if front is lifted 1.5 - 2".
I ran 32x11.5's for the life of the truck when I had a '93. I did have different wheels with the tires so can't exactly say anything about rubbing,but back then, you'd constantly see 32 and 33's on factory wheels. It may depend on your factory wheel though.

I don't remember what the 4" manufacturers suggest but that is easy enough for your to determine (go to a lift manufacturer site and look for their specs/recommendations).
I did eventually install a 4" Rancho kit but kept the 32's. Maybe 33's would have looked better but I never needed new tires after I installed the lift.
I do not know if Rancho redesigned the radius arms, but after the lift I would rub near full lock. If they redesigned the radius arms to provide clearance then you could still do 33x12.5. Factory wheels again may be an issue as they are tucked in a little.