WARNING: Settlements are already really janky. High population settlements just get more janky. You may have issues assigning jobs and what not. I had issues with this even at vanilla settlement sizes, so I assume the problem just gets worse. Hopefully, when Bethesda releases some patches that fix settlement issues it helps this mod's stability as well.

Sets the iWorkshopSettlerPopulationMax to a higher value. Options of 20, 30, 40, 50, 100, 500, and 999. I personally use 50. Managing that many people will be a bitch anyways. You can always upgrade later and I don't know how/if this affects the rate at which settlers arrive. Recommend use with 
Longer Power Lines.
Also recommend looking at Homemaker - Expanded Settlements and/or Settlements Expanded!

The default value is your Charisma level plus 10, so when you change this, I assume it will actually be Charisma+50 or Charisma+100, etc.'

v1.0 - Main release
v1.1 - Added NoObjectLimit.esp, changes workshop object limit values to nearly infinite. Happy building!
v1.2 - FormIDs fixed/Rebuilt with FO4Edit. Redownload. Thanks MookittyBonnie.

[Bugs]
Some people are reporting no settlers are arriving at settlements after hitting the vanilla cap. A small work around is that you can send people from other settlements that are smaller and still recruiting.

[INSTALL]
Install with NMM

+++OR+++

Locate 'Fallout4Prefs.ini' in your 'Documents/My Games/Fallout 4' folder.
Add 'bEnableFileSelection=1' after the [Launcher] tag.
---
Place esp in 'steamapps\common\Fallout 4\Data\'.
---
Locate 'plugins.txt' in your 'AppData\Local\Fallout4' folder.
Add 'IncreasedSettlerPopulation50.esp' (or your selected option) on a new line after 'Fallout4.esm'.

My other mods:
Longer Area Respawn
Longer Power Lines
Piles of Corpses - Increased De-spawn Timer