The word size may refer to how big something is. In particular:
- Body dimensions
- Apparent size
- Body size
- Body size scaling
- Body size and species richness
- Brain size
- Family size
- Group size
- Litter size
- Size constancy
- Size discrimination
- Size-weight illusion
- In statistics (hypothesis testing), the size of the test refers to the rate of false positives, denoted by α.
- Effect size
- Sample size