Final answer:
The most common size of attack hose used in firefighting is 1.5 inches in diameter because it provides a good balance between water flow and manageability.
Step-by-step explanation:
The most common size of attack hose used in firefighting is 1.5 inches in diameter. This size is versatile and offers a good balance between water flow and manageability, making it suitable for combatting a wide variety of fires. Some fire departments may also use 1.75-inch hoses because they can carry more water, or 2.5-inch hoses for larger fires that require more flow. However, the 1.5-inch hose remains the staple due to its ease of use, especially for quick response and maneuverability in many different situations.