Throwing my hat in the ring with two points that have long been important to me.

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.

This has a serious significance on party size for a reason I feel is obvious. Fighting some boss it might be very important to bring a well designed party into the fight, with someone to tank, heal, deal with any "trash mobs" around, and someone to burn though that bosses health bar. Roles which would demand taking certain npcs. And I waltz into that boss fight with a team of 4 squishy casters, or 4 magic-lacking melee people, because that's what the story demanded. I fight this boss, or encounter, or whatever is going on on, effectively hindered because I'm playing the story and the party size wasn't big enough to maintain efficiency while doing so.

The 2nd poing is that while I understand the mechanical and balancing choice of a party size cap in a given game, it always feels stupid regardless.
Not because I think the balance choice itself is bad, but because of in-universe (story/cannonical) context.

In Mass Effect I had this team of highly capable soldiers, and I would go on a blatantly non-stealth mission, and only bring two of them. A few mission in the trilogy were "stealthy infiltration"-y but most of the time there was no good justification for not bringing the whole team.

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.

I mean seriously, I find this collection of people who all have tadpoles in their brains, and none of them are willing to do the only intelligent thing of working together as one big group until the problem is solved.
"So we both have worms in our head trying to turn us into monsters, as do all these other people, but instead of working with you or trying to solve this problem myself I'm just gonna go stand next to your campfire like a useless fool, because I can't count to 5."

I fully understand that a dev has to pick a party size to balance combat around, but for the story and these npc's to make any sense at all, shouldn't that party size be the number of companions available?

"Hey Gith lady, I know you said your people straight up have an established cure to this ailment we share, and you know of someone who can help us reach your people, making you obviously my best hope of getting through this alive, but I'm already a fighter and I'd rather bring a guy who tried to bite me in my sleep because he's a rogue, so go wait by the fire being useless."