Set default Finder window size
So, it seems a year later, nobody had been able to solve this one, (couldn't find any solutions posted any website) so I decided to give it a shot. After some extensive research, I've arrived at the following solution.
macOS Version: This was tested on macOS 11.0 Big Sur and on macOS 10.15 Catalina as well
Customizing the default dimensions of the Finder window

Open the Terminal application, enable the &quot;Quit Finder&quot; menu item
defaults write com.apple.finder QuitMenuItem -bool true; killall Finder


In Finder, open a new window and configure its size/dimensions to the exact way you like it

Quit the Finder application (⌘ Q)

Launch the Finder application

When you first open the Finder application, a window is not created, which is a bit unintuitive ¯\_(ツ)_/¯


In Finder, create a new window (⌘ N)

The position/dimensions of this window should be exactly the way you like it. Finder determined what the position/dimension ought to be using configurations stored in the ~/.DS_Store file. Every time a Finder window is moved or resized, the ~/.DS_Store file will be written to.



Any future windows should default to opening with this size/position on the screen.
Note: If you try to open a Finder window on a larger screen, don't expect the window to be centered. Finder doesn't store the relative offset of the window to the screen, it stores the X/Y pixel offset from the top-left corner
Note: Don't try and be clever by removing write access to ~/.DS_Store, it didn't work out for me. Finder's reaction to losing write access is to ignore the ~/.DS_Store file completely, causing every new window to open with the factory-default window configurations