How to set default Finder window size for *all* situations?
I search a lot in com.apple.finder.plist file (under the folder ~/Library/Preferences) to found the answer but I can't.
I think the best option to do what you want is to create an AppleScript and use it as a service with automator.

I know that is not what you want, but it's a solution to change window's size for the active window with a shortcut.

We will use the Applescript below (more about this Applescript here)

on run
    tell application "Finder"
        activate
        set bounds of front window to {0, 100, 490, 248}
    end tell
end run


How to create the Service with Automator


Open Automator and Select Service


Search for AppleScript and double click on Run AppleScript
In Service Receives set to no input in any application
Copy the Script below and paste it in Automator

Don't forget to save your service (cmd+s)



Assign a shortcut for your service


Go to System Preferences / Keyboard / Shortcuts
From the left sidebar select Services
Find your service and add a shortcut (e.x. cmd+shift+w)



Now open your folder, press cmd+shift+w and the window's bounds will be change to {0, 100, 490, 248}

More Informations about BoundsProperty, to create your own here



Update:

Here's the AppleScript to choose your own width and height for the window without changing the destination from the side of the screen. Just change myWindowWidth and myWindowHeight. Also I have some other variables in comment to understand the code. Please reply if you have any question!

on run
    tell application "Finder"
        activate
        --we take the bounds properties of the front window
        set windowAreaDimensions to bounds of the front window
        set x1 to item 1 of windowAreaDimensions
        set y1 to item 2 of windowAreaDimensions
        set x2 to item 3 of windowAreaDimensions
        set y2 to item 4 of windowAreaDimensions

        set destToLeft to x1
        set destToTop to y1
        --set destToRight to x2
        --set destToBottom to y2
        --set previousWindowWidth to destToRight - destToLeft
        --set peviousWindowHeight to destToBottom - destToTop

        set myWindowWidth to 730
        set myWindowHeight to 521
        set sameWidth to destToLeft + myWindowWidth
        set sameHeight to destToTop + myWindowHeight

        --The following line script return the bounds of the front window
        --get the bounds of the front window

        --The following line set our bounds for the front window
        set bounds of front window to {destToLeft, destToTop, sameWidth, sameHeight}
    end tell
end run