there is a lot of "it depends"
the taller the tire, the wider you can go.
example: a 70 or 75 series tire works best when it is 2 or 3 inches wider than the rim.
get a 50 or 60 series and you're now only 1 0r 1.5 inches wider.
going down to a 35 series, the tire is now equal to, or narrower than the rim.
I'm guessing you want tall & wide?
245/75-15 would be in the middle of the range, that's about a 29.5" tall and 10" wide.
TireRack dot com has spec charts. pick a size, it has width, recommended rim width range, tread width, and overall height.
I'll let the guys that have 88 Rangers tell you what fits.