Total War WARHAMMER III: How to Fix Windowed Mode DPI Scaling Clashing with Windows at 4K

This is not a necessary step. This is just to help you understand from where the issue stems from.
Open the start menu and click settings.

In the settings screen, click on system.

Windows by default should open display settings.

To the right, you should see a section called Scale and layout.
You should see something similar to the following:

As you can see, the resolution is set to 3840*2160 (usually referred to as 4K).

The setting that causes the issue ,however, is the box above it:
Change the size of text, apps and other items
As you can see, in my case it is set to 150% scaling.
If you were to divide 3840 and 2160 by 1.5 you will get 2560 and 1440 respectively.

If you read the previous section, you should have an understanding of the behavior the game exhibits due to the scaling that Windows applies to applications.

This is why you see limited resolutions in the dropdown list when you check the box “Run in window”

As you can see, the maximum resolution listed is 2560*1440 despite my screen being 3840*2160.

To avoid the behavior shown before, navigate to the game’s directory.
An easy way to do this is to:

• Right-click the game in your steam list or the game’s icon itself and click “Properties…”
  
• Click on “Local Files” then click “Browse”
  
• You should see the game directory and files listed. Right-click the “Warhammer3.exe” file and click properties
  
• Click on the “Compatibility” tab then click “Change high DPI settings”.
  Note: If you have multiple users you can click “Change settings for all users” then click “Change high DPI settings” in the next window.
  
• At the bottom of the window you should see a section titled “High DPI scaling override”. Check the box and make sure “Application” is selected.
  
• Press “OK” on all open windows to close them.

Run the game. At first it will run in a window if you had “Run in window” selected prior to closing it.
Go to “Options” then “Graphics” and you should now see the rest of the resolutions that were missing.

Select your desired resolution.