It's difficult to determine when a penis size (length or girth) is the "perfect" size.
It is a matter of tastes. Some women like lenght, some like girth, some like both....
For instance, usually people talks about 8" length x 6" girth as the "ideal size". Well, the fact is that there are many women who will feel pain if they are fully penetrated by a 8" penis.
Also, I'm 6,5" girth and, besides my wife (we have been living together for more than 10 years now) I didn't have the chance to do anal sex with none of my occasional sex partners in the past, at least at the first time we fucked.
As soon as they saw the girth of my dick, all of them refused to put it into their asses. With some of them I was able to do anal once we have fucked several times and they "get used", so to say, to my girth.
So a big girth (6" and above) might be impressive and eye-catching but also can be a hassle if you want to fuck the ass of your partner the first time.
And also, there are women who won't have any problem to put a 7" girth into their asses while there are other who will feel pain if a 5.4" girth cock fuck their asses. It depends on the woman and her tastes about it.
But, even so, if anal isn't a must for you, there's no doubt that a big girth might be "perfect" because it fills mouths with blow-jobs and provides (in theory) a deeper stimulation of clitoris and labia with penetration.
As I said, the "perfec size" varies depending on personal preferences and tastes. It's not an absolute valid for all people.