What is the biggest size tire I can out on my 94 k1500 without lifting it?

Go to a tire shop. They have books that tells them the biggest tires that will legally fit. They also may be able to show you how to get a larger one by using different shocks or a minor lift ring under the springs. Looking for better mileage or traction? They can help you. When the newer cars that came out last year and this year, the big car makers did a lot of tire testing. They found that not a wider tire gives better traction on curves, but a more harder tire that keeps the rubber down on the road. Due to centrifugal force, only the center of the tire touches the ground at high speed, and a wider tire ends up having less road contact than a more stiff narrow tire. So if traction is your reason, do you research. R and T had the article just recently. I believe GM when testing the Corvette found a narrower tire worked better for curve traction that going massive. Just some thought.

A lot of it depends on if you have the factory wheels on it or not. Aftermarket wheels almost always have a different offset, and that puts limits on how big you can go. You'll have problems with rubbing when cutting deep one way or the other. Your truck has a Torsion Bar set up, not Coil Springs, so Coil Spring Spacers are out of the question. There should be enough adjustment to level the truck with the Torsion Bars. You can buy Torsion Keys that are indexed differently, but it's not necessary. Usually on GM Trucks you'll end up having to put blocks in the back when you put new Torsion Keys in, to make it level. Take it to an Alignment Shop, and tell them you want it leveled when they align it. It shouldn't cost anymore, as it's all part of doing an Alignment. Now to answer your question, a 285/75R16 will fit fine on your truck, and possibly a 305/70R16. 305's are about the same overall diameter as 285's, but technically should be 20MM wider. Personally, I think 285's look better, and you'll definitely get better wear out of the more narrow tire. I just think really wide tires look ridiculous, but to each his own.