I have a 1994 ford f-150 (Flareside), and I was wondering how big of tires could i put on my truck when I get a 3 inch body lift, I can comfortabley put 31's on their.

 

The body lift doesn't help where the tires would rub while turning sharp, right on the radius arm. The radius arm and tire combo wouldn't change with a body lift.
My 31's are about 3/4 inch from the radius arms when turned full lock. 33's would be mighty close if not too close You might be able to get a s. swamper 33x10.50 to roll ok. I was gonna do that this spring but money has eluded me

 

I have 35X12.5 tires on my truck and the tires do rub the radius arms if you turn to tight but eventually you learn how far you can turn the wheel without them hitting. 33's might come kind of close to front bumper, but they should fit. Try superlift.com somewhere on the website they have a chart that will tell you what size tires with what lift you can put on your truck.

-Jon