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Size is a general term for the magnitude of an object or quantity, describing how large or small it is. It can be expressed along one or more dimensions—such as length, width, height, depth, diameter, area, volume, or mass—and is often compared to a reference or standard.

Sizes are defined using measurement systems that assign units to those dimensions. Common length units include meters and centimeters; mass uses grams and kilograms; volume uses liters. In specific fields, additional conventions apply: typography uses points and picas for type size; screen and display dimensions are given in inches or millimeters; paper sizes follow standard series such as A4; clothing and footwear use size scales that may be labeled XS–XXL or numeric values.

Size can be absolute, based on objective measurements, or relative, described by qualitative terms (small, medium, large) or as a proportion of another quantity. Different domains may have different notions of what constitutes the same size, leading to regional or brand variation.

Because size affects fit, capacity, and performance, it is a central parameter in design, manufacturing, and consumer decision making. Objects can be scaled up or down while preserving shape, a process that changes their other properties as needed.