The definition of a commercial HVAC unit size is the physical size of the unit, which is determined by the amount of air it can move. This size is measured in British Thermal Units (BTUs) and is determined by the size of the space that needs to be cooled or heated. A commercial HVAC unit size can range from small units that are used to cool a single room to large units that are used to cool an entire building. The size of the unit will also depend on the type of system that is being used, such as a split system, ductless system, or rooftop system. It is important to choose the right size of commercial HVAC unit for the space that needs to be cooled or heated, as an undersized unit will not be able to adequately cool or heat the space, while an oversized unit will be inefficient and cost more to operate.