True size of Africa
We stunningly under-estimate it.
A sphere cannot be represented on a flat plane without distortion, which means all map projections distort in one way or another. This means your map gives you a very weird sense of size.
No where is this more apparent than the actual size of Africa. The image below is a few years old, from the Guardian http://goo.gl/tdUYbp
, but makes my point really nicely.
Imagine how many other perceptions we have of our own size are misplaced. :)