Four party members is classic DnD video game size. Considering how cake fights have been so far with just four, I can't imagine how broken six might be.
All the classic D&D games I remeber where six party members, the Eye of the Beholder games was four player created and two NPCs, the Baldur's Gate games where one player created and six NPCs, the Icewind Dale games where six player created, I never played Temple of Elemental Evil but I believe that was six characters as well and most of these have been held up as the gold standard for various people over the decades.
