When and how big?
I'll be installing another transmission cooler shortly and figured while I'm in there maybe to install a power steering cooler (if necessary)
I'll be running 33's max (for now) and full armor + winch.
I've been running Mobil 1 ATF in my power steering system, since I went through and changed all of the factory fluids to Synthetic, at 5000 miles. I've always run LT265/75R16 (32") tires, well from about 300 miles on, anyway. Even in some brutally hot 120+° F temperatures in the desert southwest in summer, I've had no overheating power steering fluid issues.
Now if we were talking one of those crappy, whining, Ford power steering pumps, I'd suggest synthetic fluid, and an oil cooler.