The recently-released Witch 3.5 includes a feature that pops up a preview window when you leave a window selected in Witch's switcher panel. If you find the preview window too large (or too small), you can modify its size with a hidden preference. Disable Witch first, then run this Terminal command:
defaults write ~/Library/Application\ Support/Witch/Settings "Preview Size" -int 20The 20 represents the size of the preview window in relation to screen height; Replace that with a desired size, ranging from 10 to 100, then re-enable Witch. The default setting is 40, and values over 80 are not recommended.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20100613145625238