Size is the most fundamental structural variable of the universe. Neither bacteria nor empires are immune to its laws. Measuring it is challenging, especially where complex systems like economies are concerned, yet mastering it offers rich rewards: the rise of the West, for example, was a direct result of ever more accurate and standardized measurements. Grounded in history and drawing on the latest science, with much recourse to art and classic literature, ‘Size’ explains the regularities – and peculiarities – of the key processes shaping life (from microbes to whales), the Earth (from asteroids to volcanic eruptions), technical advances (from architecture to transportation), and societies and economies (from cities to wages).