Back in late 2013 the comment that “Standing shoulder to shoulder the world’s population can fit on the Isle of Wight” became the inspiration for a work-in-progress under the working title of Megalopolis.
The Isle of Wight is a island just off the southern cost of England, and whilst the comment might have been true in the 1920s when the thought experiment was first floated, it certainly isn’t today. I reckon we’d each have a 20 cm square to ourselves. That would not work for me!
If we did want to move the world’s population to a single mega-city, say one of a similar population density to London, then this mega-city would cover some 1.4 million sq km and be around 1,300 km from edge to edge. What is perhaps enlightening about this is how small the area is on a global scale.
This hypothesising is interesting, but more or less useless for developing story. It’s just world building, and stories don’t come from world building alone.
Stories are about characters with unfulfilled needs and their quest to fulfil them. The conflicts that stop them fulfilling their need may well come from the world it’s set in, but without characters and needs it’s just an empty space. Writing advice has told me this for years, but it took another three years to sink in – for me to stop world building and start making people. So, Megalopolis is based around people with problems living in a super-mega-city, where apparently their problems could be solved simply by getting out it.