How to set default finder window size?

It is very easy to do this:

1. Open a new Finder window.

2. As you resize the window, hold the Command ⌘ (not Alt / Option) key. Close resized window.

3. Then hold the Alt / Option key as you right click on Finder in the Dock and click Relaunch.

4. Any Finder windows opened after this will open at that size.

Note: This does not work for windows opened from folders on the desktop.

This works for me in MacOS Catalina 10.15.2:

1. Open a new Finder window;

2. Resize the window to the size you want (however don't move the window to your desired location yet);

3. To lock the location and size:

   1. start by pressing the Command ⌘ and Alt / Option keys together and drag the window to your desired location;
   2. release your mouse button first then release the Command ⌘ AND Alt / Option keys second;

4. Close resized window;

5. Any Finder windows opened after this will open at that size.

Open a finder window, hold down the option key and resize the window as you wish. Close all finder windows and next time you open a new one it will remember the size.