Loud and soft, dark and light, big and small. Each is a relative measurement of some quality on some scale. Day doesn’t exist without night and up doesn’t exist without down. Each exists in contrast to the other and contrast occurs on a scale.
In design when we talk about scale we’re usually talking about the quality of size, bigger and smaller, and the relationship of different design elements at different sizes. We’re also talking about proportion, which tells us how well we’ve scaled our design.
Size, scale, and proportion are all related concepts. Let’s define them quickly and then talk about each in a little more detail.