5" would be ideal. 4.5" will probably have the tires rubbing the fender lip, 5.5" will probably have the tires rubbing the inner fender. If your only option is 5.5", you can run a spacer to get it centered in the opening.
This is assuming a 275/60 15, the most common rear tires for the 3rd and 4th gen Novas. You may be able to sneak a 245 or 255/60 with 4.5" backspacing.
I would not put 15x8's in the front, they are going to stick out and hit the fender lip if they have the right backspacing to clear the inner fender on the inside. The track width on the front of our 4th gen Novas is wider in the front than the back, so it's preferable (visually) to put a smaller wheel/tire in the front anyway. I'd recommend a 15 x 7 with 4.25 or 4.0" backspacing in the front.