First of all, regardless of what that party size is, my choices of who to bring to any given part of the game, is going to be almost entirely if not literally entirely based on story reasoning. This is true of any game with choice of companions but especially true of highly story driven rpgs like this.
I'm not going to bring the wizard and fighter to fight the boss for the well designed loudout for the difficult fight, I'm going to bring Gale and Lae'zel to fight the boss because they both have some sort of personal history with them. It never mattered what class my Shepherd was in Mass Effect, what the enemy type was going to be, or what combinations used the "detonation" mechanic. I brought Garrus on Garrus missions and Tali on Tali missions.
Unless it's been changed recently, I think Larian has said that our choice of party members will be locked after Act 1. Presumably, the companions you don't choose will go their separate ways from camp to find a solution to the tadpole, and you'll carry your chosen 3 companions through the rest of the game.
This is similar to how DOS2 worked, so not surprising, but I know some people won't like it. I don't mind it myself, because it means I can form a closer bond with the chosen adventuring party. It also gives me a reason to play through a second or third time with different companions.
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Here in BG3 the same thing that happened to me in DoS 2 keeps happening. I meet some npc, who has a great reason to work with both me and the rest of my party, says "yea I'll join you, but only if there's no more than this many people around" and my immersion is immedieatly completely broken.
If it's true that the party will be locked after Act 1, then we won't be seeing that kind of dialog after that point. There won't be any other companion choices after that.
