After experimenting for a bit, it does seem like the extra mulligans allow for slightly larger deck sizes. I only used 25 card decks in Closed Beta, but since transitioning to Open Beta I've been running both of my decks with 26-28 cards.
I've found I have had more luck with 28-30 cards then in the closed beta. The extra mulligan really changes the game around in interesting ways. Plus the card synergies makes for some interesting changes. I have a feeling a low 30 deck will become the standard now, unless the deck in question relies heavily on drawing key gold and silver cards.
I'm using a nilf Emissaries deck atm (due to lack of better cards) and i'm running 28, on top of standard 25 i have one clear skies and two standard bearers, you have no idea how many games i won only because of those 3 cards clearing weather effects.