You could find dorm rooms as small as 100 square feet (a small single) or as large as 300+ square feet (a larger double or suite room). A typical double room might be around 150-200 square feet.
To find the size of a specific dorm room, you'll need to check the housing information on the university or college website you're interested in. Look for floor plans or room dimensions in their housing brochures or virtual tours.