I honestly doubt that party size will affect amount of players able to join via multiplayer ...
That would require A LOT extra work to implement. :-/

I dont quite understand what penalisation for smaller party are you talking about ... that would aply for when game would be ballanced for 6 members, yet you would like to go with only 4 ... but our situation is other way around.

Game is prepared for 4 characters ... encounters are all created for 4 characters ... every game mechanic is "ballanced" (for lact of better term) around 4 characters ... so nothing really changes compared to what we do have now.

Yet, if allowed, you would be able (just able, no need to do that if you are not comfortable with such big party, game is still prepared for 4 characters) to take another 2 with you to make party of 6, instead of intended 4.
Yes, it would make your game easier (more carry weight, more spells, and spellslots per rest, more actions per round, etc. etc.) ... and that is exactly why i said, it might be seen as part of difficiulty settings. laugh

I didnt play Pathfinder, so i cant really be judge of that ...
But i really love their difficiulty settings:
[Linked Image from spritesanddice.com]
In matter of its not just "easy / normal / hard" ... but you are able to adjust thigns the way you like it ... in our case that would mean things like amount of resources needed for long rest (since many people here is still complaining that food does not limit Long Rests enough ... so they could double the needed amount of food, to limit themselves as they see fit), carry weight (since many people here is still complaining about the fact that you can carry more than single Barrel, if your character have enough Strength ... so they could halven their carry weight, to limit themselves as they see fit), and the other stuff.

I still dont understand why cant we change Race for our hirelings. frown
Lets us play Githyanki as racist as they trully are! frown