I think a party of 5 would actually be the ideal. Let me illustrate based on my experiences so far with the game. I'm playing a ranger and thanks to the way I built her my character can use thieves tools to open locks and deal with traps (though I haven't encountered any traps that I can disarm, maybe I'm just being dense and missing when I can do this) so I don't need Astarion in my party for that. Lae'zel has been my favourite character thus far and has been a reliable tank from the beginning. Likewise Shadowheart has been absolutely vital as a healer. The limit on short rests and the relative scarcity of healing items makes it so, and it's rare I come out of a big fight not desperate for healing, and the game has done a good job making me hesitant to take long rests (I think I've only taken 4 in the past 12+ hours of playing). Plus she hits hard enough that she serves as a good frontliner alongside Lae'zel. I would not last long without Shadowheart for healing so she's a mainstay. That leaves me with only one extra slot for another character. Now consider that the
goblin camp
is a really important area for Wyll's storyline and not taking him means you're gonna miss out on what seems to be a major beat in his story, so I have heavy incentive to take him along. Which means that for the goblin camp my party is set from the get-go since without Shadowheart and Lae'zel I would not be able to handle a lot of the combat without them should I get into combat (and I accidentally aggro'ed the whole camp so having them is the only way I could play and not die).

Now I like all of those characters so it's not the worst situation in the world, but right now with the characters I have currently, I only realistically have one free slot for the remaining 3 companions. A fifth slot would immediately allow me to take another non-essential companion who I want to have along purely for story reasons. A lot of people hav been talking about combat length but the only time I've felt like combat was too long was when I accidentally aggroed more enemies in the middle of a battle that had already been going on. So personally having the combat be longer would a plus for me since I enjoy it a lot overall. Plus a fifth companion would mean that while combat length might be greater, each character in the party would be expending relatively fewer resources so you likely wouldn't run out of them as quickly.