Mini-split air conditioning units come in a variety of heating and cooling capacity sizes. The smallest size is the 9,000 BTU unit and the largest size is the 24,000 BTU unit.
The 9,000 BTU unit is ideal for a space between 100 and 200 sqft. Typically most bedrooms fit into this category.
The 12,000 BTU unit is ideal for a space between 200 and 400 sqft. We recommend using this capacity size for master bedrooms, small living rooms, and kitchen areas.
The 18,000 BTU unit is ideal for a space between 400 and 700 sqft. We recommend using this capacity size for larger living spaces (kitchen + living).
The 24,000 BTU unit is ideal for a space between 700 and 1000 sqft. These units are typically used in retail stores, office space, and restaurants. Depending upon the size of the space and the number of occupants, you may need multiple units to adequately supply the heating and cooling needs.
In order to maximize occupancy comfort and energy efficiency, it is important to select the right unit for your home, office, or retail needs.
Reference
http://mitsubishiacdealers.com/info/what-size-system