In OS X Lion the thumbnail previews of windows from the same app are grouped together. By hovering the mouse over these thumbnails and scrolling up, you can expand the group of windows. It is possible to display these windows in full size.
defaults write com.apple.dock expose-cluster-scale -float 1
Restart the Dock to make changes affect.
The value 1.0 equals 100% window size, you can adjust this for example 0.6.
Restore to the default settings with:
defaults delete com.apple.dock expose-cluster-scale