On the older cars, they tended to be fairly wide. I'd look on line for pictures of a 1970 Trans Am - the racing versions were white with wide blue skunk stripes. You'll have to guess at the size though.
On my Neon, I found that 8" wide and 4" apart was about right. On the Nova, you may want to up that a little; say to 12" wide and 6" apart.
They definitely want to be closer together than the width of one of the individual stripes. Hope this helps!