In my estimation, it depends on your relationship status and how broad your dating variety is:
If you have a long-term significant other --> any
Girth that exceeds her threshold (of pain and "fit").
If you are single and date mostly under 30's with no kids, I'd say 5.5" is prime, and anything above will have diminishing returns.
If you are single and date the whole field of availability, then I'd say 6.0" is prime, and anything above will have diminishing returns.
This is based on both my experiences and tons of anecdotal information. I should note this applies to a hetero male, and I imagine it's a different standard among gay men from what I've learned in the past.