The previously-covered “Move Outlines” site let you trace a border around a geographical feature, then compare that border size with any other area in the world. The BBC’s Dimensions website (aka How Big Really) is a nice companion site; while it doesn’t let you draw your own border, it has pre-drawn areas covering a number of different topics, like Environmental Disasters:
Ancient Worlds…
Festivals and Spectacles …
… plus Space, Depth, The War on Terror, The Industrial Age, Battle Of Britain, and Cities In History. Great for getting a sense of scale.