The Earth is a messy place with irregular shaped continents covered in all sorts of different stuff: cities, woodland, crops, etc. So I've tidied it up by collecting all the different types of land cover together. In the picture below, different types of land are gathered into circular islands whose area is equal to the total area of grassland, forest or whatever. In the picture below that, I've gathered all the land together into one super-continent and divided the whole continent up like a pie-chart of land cover.
It turns out that all the World's urban areas could fit on an island just 616 km across. Over 50% of us would live on that island!
See also: Allotment - what would we each get if we shared the world out fairly?
Data: IIASA (http://www.iiasa.ac.at/Research/LUC/GAEZ/tab/t44.htm)