As far as "box" wine, we brought a 3 liter box of wine on last year's Carnival Legend and Golden Princess cruises. We bought the boxes at Winn-Dixie across from Embassy Suites in Fort Lauderdale and, since you could bring wine on board, just put it in our carryon. As far as a box being larger than a bottle, no mention is made in the cruise brochures as to how large a bottle you may bring on board. Here are some wine bottle sizes and their equivalent in 750 ml. The box wines are great to pour from for a quick glass of wine on your balcony.
Normal Bottle 750 ml
Magnum 2 bottles
Marie-Jean 3 bottles
Jeroboam 4 bottles
Reboboam 6 bottles
Methuselah 8 bottles
Salmanazar 12 bottles
Balthazar 16 bottles
Nebuchadnezzar 20 bottles