Paper sizes refer to standardized dimensions used for printing and writing. The most common system is the ISO 216 standard, which includes sizes like A4 (210 x 297 mm) and A5 (148 x 210 mm). These sizes are widely used in many countries for documents, letters, and publications. In the United States, a different system is prevalent, with Letter size (8.5 x 11 inches) being the most common. Other sizes include Legal (8.5 x 14 inches) and Tabloid (11 x 17 inches). Understanding these sizes helps in selecting the right paper for various tasks.