15" Ralleys

I just got the ebst kind of parts, two free 15" rallyes. I had thought son swappingn to cragers all around to get some stance on the car but I have nothing against the rallyes. I've seen it done with 15" in the rear and 14" in the front. I currently have 205/70R14 in the front I wanted to put an 1" or 2 stanc eon the car so what size would work well on the 15" wheels on the rear? And how wide? they look like maybe 7"