Some facts about party size in D&D 5e, just to make sure we're all on the same page. If I get something incorrect please correct me.
  • DMG pg 83: CR and encounter building "assumes that you have a party consisting of three to five adventurers." Specifically, 3 to 5 players are treated as equivalent, whereas 6+ and 2- require additional calculations to determine encounter difficulty.
  • All Adventure Modules suggest either 4, 4-5, or 4-6 players. No modules that I'm aware of suggest 3 players.

In summary, while all party sizes are technically allowed, the standard seems to be 4-5 players with 6 or 3 less common but still explicitly encouraged by official D&D books.

Personally: +1 for allowing a larger party size where split exp is used to balance parties of all sizes, as long as it's an option in settings and not in-game. I want the base game to remain balanced around a party of 4 and I don't want to, by default, be allowed to have 6 people in my party.