How much bigger tires can I go on my stock 1991 ford f150 4x4?

I run 33x12.5 on my 88 with only some tire contact at full lock.

My 88 F150 had 32 x 11.5/15R - if you are only concerned with appearance they looked big, but they rubbed before full lock, and made getting in a struggle without a step bar. Going back to stock 235x75 made a big difference in getting in. I was surprised that the speedometer was hardly affected by the change. The ride height diff. was just about 1.5 inches.

my 92 f150 has 31x10.50/15 on it and it ran great before i put a four inches lift.