There is no information on an internationally average size.
400 square feet.
240 square feet
525 square feet.
162 square feet.
300 square feet.
120 square feet.
252 square feet.
16' x 15' = 240 square feet.
A simple rectangular room 20 feet by 18 feet has 360 square feet.
A 10x14 room has an area of 140 square feet (10 feet x 14 feet = 140 square feet).
An average sized room such as a bedroom could be ten feet square or eight feet square. Larger rooms will be longer than they are wide.
There are 270 square feet in a 15'x18' room.