As Expanded party size mod https://www.nexusmods.com/divinityoriginalsin2/mods/24?tab=description  is not being updated for some time, i've done some reverse engineering. So if you like you can repeat it in divinity engine 2 to make it yours. or you can download this. His mod idea is great and i'm only sharing it as it was not updated for the last 2 main updates.

Please make sure to check the original mod and give him your endorsements. I only repeated his idea into new release.

News:
1.2 version is released. This is re-compilation of the mod for Hotfix v3.0.171.819  Patch6 hotfix2
I've checked it over an existing save by removing and re-adding the team. It is best to collect feedback if this solves the issue for people...
Thanks to Dark Messiah who also confirms it in a new game.


Advice: Use it in a new game . 

IMPORTANT : The mod has changed the "STORY" for single user. When i check the story files, the initializations are quite different for Arena and GM Modes. If you want to use it in Arena or GM Modes, it will not work. 
Compatible with:  Hotfix v3.0.171.819  Patch6 hotfix2



Installation: Unzip  to Documents\Larian Studios\Divinity Original Sin 2\Mods

Note version 1.2 is released only for the Expanded_Party_Size_Updated. I don't see the need to continue 3 options while none of them has a blocking bug...  

The part below applies if you want to use the old version of the mod for patch 5...

WARNING: Do not have all 3 options in your mods file. They will override as they have the same mod id. keep only one of them in your mod folder.
Selection of which version of the mod:

1) Expanded_Party_Size_Updated V1.0-277 :Full mod with all the changes described in Detailed Description. 

2) Expanded_Party_Size_Updated base-reflection-dialogue :This is equivalent of THE DIALOGUE FIX VERSION of the original mod for patch 5 hotfix 1&2. I've removed the additional change i did:
DB_PartyDecisionDialog_PlayerIdentifier //add
DB_PartyDecisionDialog_PlayerIdentifier(5,"PDD_Player5");
DB_PartyDecisionDialog_PlayerIdentifier(6,"PDD_Player6");
DB_PartyDecisionDialog_PlayerIdentifier(7,"PDD_Player7");

3) Expanded_Party_Size_Updated base-no-dialogue:This means only your first 4 characters that joined the team will make comments. you can have a team of 7 characters. last 3 will not make their comments in party dialogues. Please use this as safer alternative if you are not interested in party dialogues any way.

Test Status: Fort of Joy, created a team of 7 succesfully. Also observed comments from all team members for Dallis event. Still, there is a part in reflection dialogues where there might be an issue with only hearing 4 team members. 
I've not tried with 8 characters. i assume it shall work but as you can see in the edits below, 8th character will not be part of party dialogues.
During testing i can see that dialogues are triggering ProcDefineReflectionDialog("FTJ_SW_RD_VoidlingAmbush",_Player); and similar to initiate a reflection dialogue. In my tests, i could not come up with a consistent solution that works for all. in some dialogues, you will observe 4 party members to comment in others all party members. Make sure to recruit the first 4 to be the ones which you want to hear their reflection comments. I could not observe any major issue that affects gameplay apart from this. 
In game, There are 51 Reflection dialogues and 14 PVP dialogue possibilities. I've observed the first 6 before escaping Fort Joy. Out of 6 , 2 woks for all characters, the other 4 took only the first 4 characters into account only.
Sadly, Dialogue structures are the same and the calling Proc command is also the same. I just can't see a consistent reason on why it worked for some but not for all...

Known Issues:
1) a bug with party responses. It will be game affecting. It will happen if you use the mod with an on-going game. 
Solution: Use the Mod only with a new game
Workaround if the issue happens: Ienin was kind enough to post his workaround in the forum:
The only current fix I have at the moment is disabling the mod once you recruit your companions. Just don't dismiss them, and they will still function as normal. You will need to re-enable the mod between acts most likely. For sure, at the start of act 2.

The other alternative is to save at the start of every reflection dialog, disable the mod if you can't end dialog, reload to finish the dialog, and then re-enable the mod again. Both are kind of annoying, but it'll cost you 3 minutes (approximately) of your time if you want to play now
2) Party Full Error for a co-op game
It is reported but there is not enough information. So please report back how it plays ladies and gentelmen if you use the mod for a co-op game

Detailed Description: "If you want to create this mod yourself"

 1. create an add-on project as "story" selected in divinity engine 2 editor. don't open up any level. go from the menu and select story editor. close and save.
2. via file browser gointo your new mod location, copy these files under "story/rawfiles/goals in your new mods folder 
GLOBAL_PartyDecisionDialogs.txt
GLOBAL_ReflectionDialog.txt
GLOBAL_PartyNumber.txt
GLO_Origins.txt
(you need to extract the pak files and all patches in order to get them)
also copy these files from /story folder to your new mod directory
goals.div
goals.raw
story.div
in notepad++, search within each of these files. i define the action after // and define what shall be added or replaced

Search parameters
GlobalSetFlag("GEN_MaxPlayerCountReached"); //replace GlobalClearFlag("GEN_MaxPlayerCountReached");
DB_Origins_MaxPartySize(4);   //replace  DB_Origins_MaxPartySize(7);
PartyAmountFlags //add 
DB_GLO_PartyAmountFlags("GLO_PartyOf7", 7);
DB_GLO_PartyAmountFlags("GLO_PartyOf6", 6);
DB_GLO_PartyAmountFlags("GLO_PartyOf5", 5);
DB_ReflectionDialog_PlayerIdentifier //add 
DB_ReflectionDialog_PlayerIdentifier(5,"PDD_Player5");
DB_ReflectionDialog_PlayerIdentifier(6,"PDD_Player6");
DB_ReflectionDialog_PlayerIdentifier(7,"PDD_Player7"); 
_NewCount <= 4 //replace NewCount <= 7
// Note: below change is new compared to the original mod. I see the hesitation of the original author for this part as there is still a weird script routine where i'm not sure if this change alone will fix. still, till now, this change worked  fine for me
DB_PartyDecisionDialog_PlayerIdentifier //add
DB_PartyDecisionDialog_PlayerIdentifier(5,"PDD_Player5");
DB_PartyDecisionDialog_PlayerIdentifier(6,"PDD_Player6");
DB_PartyDecisionDialog_PlayerIdentifier(7,"PDD_Player7");

After doing the changes, open your mod agin in the divinity engine 2 editor and after going into story editor, try to compile your mod. if it is succesful, you should try play testing.

With instructions above, when you load the mod into game, new story .osi file will be generated by the game engine
so in case, you want to create the mod yourself in the future, you can...