I had a problem where somehow a Finder window was shown so large that I could not grab the bottom right corner and resize it. I searched and found that you can’t fix this in OS X. You need to download a third party app instead:
When you install it, run it then select the window you need to fix and hold down control + option and then use your arrow keys to move it around to where you can get to the corner of the window again.
Maybe this will help someone.