Stock, 31". TT/shackles, 32", maybe 33" if it isn't too wide. Key is the rim offset, if you're getting 15" rims it's easy to find 15x8's with a 3.75" backspace which is what you want. 16" rims are tougher, the least backspacing I've seen on a 16x8 is 4" on a Pro Comp 1069 wheel.
With a 15x8/3.75" BS a 32x11.50 should fit fine. BFG does make their A/T and M/T in a 33x10.50-15 size which may fit with that rim.
On 16's, a 265/75-16 is about a 32x10.00 and a 285/75-16 is roughly the same overall size as the 33x10.50. Again, the problem with the stock rims and aftermarket 16's is that they usually have too much backspacing and bigger tires will rub on the inside. I think that spacers are generally a bad idea and in my state spacers over 1/4" are illegal anyway.
Lastly, if you're buying a whole set 15's are a bunch cheaper than 16's for both the wheels and tires.