A recent problem that I've faced is that Nautilus (the file manager of Ubuntu) was resized and I was not able to change the size of it.
This problem also applies to other applications which can't be resized.
Take a look at how it was:
The fixes for this problem are:
Press ALT and then middle click (mouse) on the window of the file manager and then resize.
Note: Your cursor must be at the edge of the window for this to work.
Click on the files window and then hold ALT and press SPACE . A small menu will appear at the top left corner of the window. It will be like this:
Click on resize and restore it to the original size.