What it artistically symbolised in Greek culture 2,000 years ago doesn't matter, what's actually relevant to the question is what penis size actually affects.
The obvious one, as suggested by modern stereotypes, is ability to please women. There's actual studies on that, verdict is IIRC "slightly longer than average, but circumference is more important, it's got to be fat".
The next obvious benefit, and probably the bigger one, is that because our culture's modern stereotypes about penis size suggest bigger = better, having a big penis and especially knowing it's bigger than other peoples' dicks is a boost to confidence/ego/self-esteem.
That being a status game thing, the answer with respect to those terms is "bigger than any of your rivals' penises". Or possibly "world's biggest", but the current world record holder has enough problems from it that the trade-off probably stops being worth it way before then.
So I'd say "world's biggest healthy functional penis", half an inch longer than Jonah Falcon's.
The problem then is, once genetic engineering comes along we'll enter an arms race of monstercocks, but that'll probably ruin the "status symbol" quality very quickly and turn it into a fashion thing.
What's the best penis size? What's the best shoe style? It's just a fashion statement unless you're not rich enough to stay up-to-date on the latest dick-styles.
