MapFight is a fun little online app ~ anonymous ~ that let's users compare the sizes of different countries and U.S. states. It's very useful in giving a sense of the sheer size of foreign places when users lack experience with them. For example, Maine is revealed as being the same size as Portugal.
The issue, of course, is that the maps we use to visualize the world all distort shapes or areas, or both, so that it can be hard to make direct comparisons. It will be of most use when thinking about areas in the tropics, which commonly used map projections generally show undersized. Mali, where French and US forces are currently intercede against an insurgent rebel force, is not some small, minor place but is almost twice the size of Texas! Syria, wracked by vicious civil war, is the same size as Wisconsin.
h/t Max Edelson