I found this solution works - at least for now.
It is a short AppleScript that measures the bounds of a window and then calculates the new bounds by leaving the top left corner where it was and also retaining the bottom edge position. It just shrinks the width.
I keep stuff like this in an XMenu item where I can get at it easily. If you save it as an application from the script editor, you could drag that app to the dock. Or probably the best solution would be to attach it to a function key either with a keyboard short cut or some kind of service call.
Note that this operates on the frontmost Finder window only.
Using this technique you can still set the Finder windows to silly values like 20 pixels but the Finder then gets confused. As soon as you manually resize the window, Finder will snap it back to its minimum size.
Now the question is, how long will it be before the Aple engineers spot this and intercede when you set the Finder Window Bounds and stop this trick from working. I can rationalise why they might want to make sure you cannot create a zero width Window but the current minimum size is annoyingly large.
I have left out any sidebar and task bar switching. Its a good idea to deal with that separately with Command-Alt-S and Command-Alt-T keyboard shortcuts.
Anyway, here is the script:
tell application "Finder"
set myBounds to bounds of front window
set myLeft to item 1 of myBounds
set myTop to item 2 of myBounds
set myRight to item 3 of myBounds
set myBottom to item 4 of myBounds
set bounds of front window to {myLeft, myTop, (myLeft + 200), myBottom}
end tell
