The scale of a map shows the ratio between the distances on the map and the corresponding distances in reality. For example, a map of scale 1:50,000 shows a distance of 50,000 cm (or 500 m) as 1 cm on a map. A 1:25 scale model of a building with a height of 30 m has a model height of 1.20 m.
Scale bars for microscopic or unusual objects are necessary. They help readers to understand the diagram.