I have 15" wheels on my 3/4 ton. I am currently sporting a set of Cooper 235/75R15 tires that are almost 29" across (Seems like they're 28.87 or something).
Modern radial tires come in several different aspect ratios (ie the "75" part) If you were to go with a 255/80R15, you'd end up with a 31" diameter tire. The only thing is, you're looking at 10 inches wide. In comparison, the 235/75R15's that I'm running on mine are 9.25" wide.
16" wheels are another story. You can get 235/80R16's that will be 9.25" with a diameter of 30.8".
I hope this helpes.
