How to set default finder window size?
It is very easy to do this:


Open a new Finder window.
As you resize the window, hold the Command ⌘ (not Alt / Option) key. Close resized window.
Then hold the Alt / Option key as you right click on Finder in the Dock and click Relaunch.
Any Finder windows opened after this will open at that size.


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