Cannot set permanent Finder window size
The common trick stops working when you open a new Finder window starting at a folder you've never viewed before—a location that doesn't have any Finder window info saved. You could replicate this by making a new folder on your desktop and then double clicking it to open in a Finder window. You'll see the size has been reset.
I'm pretty sure the &quot;real&quot;, permanent Finder default is hard-coded into the Finder binary. I've seen stuff hinting towards that within Hopper.
Here's the next best thing—you can set up an Automator service that quickly resets the front Finder window to your preferred size:

Open Automator, and create a new Quick Action / Service.
Set &quot;Service receives no input in Finder&quot;
Add a &quot;Run Applescript&quot; block.
Paste in: tell application &quot;System Events&quot; to tell application process &quot;Finder&quot; to set size of front window to {770, 460}

Edit {770, 460} to your preferred width and height, respectively. Note that you can find a Finder window's current size by running the following in Script Editor: tell application &quot;System Events&quot; to tell application process &quot;Finder&quot; to get the size of front window.
When you're done, save your Service with a name like &quot;Reset Finder Window Size&quot;. You can also go into System Preferences &gt; Keyboard &gt; Shortcuts &gt; Services and assign your Service to a keyboard shortcut.
The first time you run the service, it will likely fail. To fix, open System Preferences &gt; Security &amp; Privacy &gt; Automation and grant Finder the ability to control System Events.