Here's another great question? Biggest dually tire size on stock rims
 

Ok. I'm sure someone else would like to know this, or has asked this, but I couldn't find it. What's the biggest tire size someone can put on a stock 3500 dually 16 wheel?

That will be determined by which tire brand you decide to go with. I wouldn't recommend going any bigger than a 35 on it. 38's would look a little dorky!

I think what you mean is how big can you go with the stock height and stock rims?
-For that, you can only go to a 31x10.50 without hitting anything. What do you want to run?

255/85r16 (33.3x10), will fit stock rims. 235/85r16 (32x9.3), should be stock size.