Shrink the Comment Window
The Problem:
If you add a comment in Normal view, Word opens an absurdly large comment window at the bottom of the screen. It takes up far too much screen real estate, even on my 19" monitor. To make matters worse, you can't close the window by clicking on an X. You have to go to the menu bar and find the option to close it.
The Solution:
As you say, Word does tend to display the Reviewing Pane at far too large a size. Drag its split bar to slim it down a bit. To close the Reviewing Pane quickly, drag its split bar all the way to the bottom of the screen, or simply double-click the split bar. You can also press Alt+Shift+C to close this pane (or any other pane except the task pane, which requires Ctrl+F1).