15x6 is going to limit your choices, mainly on width. Other things that need to be considered is the final drive ratio in the rear end, as well as what you want the final ride height to be. Tire load ratings also need to be considered, the shorter the sidewall, the less load the tire can safely handle.
With the information presented so far, though, a 215/75/15, or perhaps a 225/70/15 would be pretty good on the wheels you mention. If you bump up to a 7" wheel, a 235/70/15, or if you want it a little lower, a 235/65/15 would be good.
