Which of the following is commonly used to estimate the size of a room?

The correct approach to estimating the size of a room involves calculating the area in square feet. This is done by measuring the length and width of the room and then multiplying these two dimensions. This calculation provides a clear representation of the floor space available in the room, which is essential for determining the capacity required for heating, ventilation, and air conditioning systems.

Using drop ceilings, while relevant for other aspects of room design such as aesthetics or acoustics, does not directly contribute to understanding the overall size of the room. Measuring the height of the walls, on its own, provides information about the volume of the room but does not assist in calculating the floor area needed for HVAC sizing. Taking the length and breadth of furniture can inform spatial planning, but it doesn't reflect the complete dimensions of the room itself. Thus, for proper HVAC analysis and sizing, calculating the area in square feet is the most effective method.