Adult shoe sizes are standardized measurements that indicate the length and width of shoes. These sizes vary by region, with common systems including the US, UK, and EU sizing. In the US system, sizes typically range from 5 to 16 for women and 6 to 18 for men, with half sizes available for a better fit. To determine the correct shoe size, individuals can measure their foot length in inches or centimeters and refer to a size chart. It's important to note that different brands may have slight variations in sizing, so trying on shoes is recommended for the best fit.