Is it necessary to use a 7.9 gallon primary fermenter if I plan to make a 2 gallon batch of wine, or will a 6.5 gallon primary be sufficient?
A 2 gallon fermentor would work. Wine doesn't foam much. There's no reason to use a fermentor that much bigger than your batch size.
To paraphrase Dr. England - "Off-flavors smooth with time. So do mountains. Brew it right from the start!"
My blogsite: http://nateobrew.blogspot.com/
To actually answer the OP's question: a 6.5-gallon primary will be fine.