How much does deck size effect how good the deck is?
I also think it depends on what the overall strategy of the deck is.
A defense-heavy deck that is built to completely shut down an opponent's deck will not be any good at 50 cards. It is HIGHLY unlikely to succeed at 57 cards. But it has a good chance of winning most of its games at 63-70 cards. Bigger than that is again unlikely to succeed unless you get up to 154 cards.
An offense-heavy deck that is built to be unstoppable and get to 5 LS consistently before your opponent is HIGHLY unlikely to succeed at anything over 57 cards, and is MOST likely to succeed at 50 or 51 cards.
A balance deck has the most flexibility, but I suspect still has the best shot at winning if it is somewhere between 50-57 cards.