Finder Window Will Not Remember Window Size Set

Once you change the window's size try moving the window away from its current location then move it back. I have found this to be a minor bug in the Finder that it does not save a dimension change until you move the window. It does save the positional change that in turn changes the size information.

Be sure you open the Finder window by double-clicking on the Desktop disk icon, not the icon in the Dock.

The disk icon that you should see on your Desktop. If you don't see one, then click anywhere in the Desktop and then select Preferences from the Finder menu. Click on the General icon in the toolbar and check the top four checkboxes.