As a general rule of thumb, most businesses allow between 100 sq ft to 140 sq ft (net) of office space per person, depending upon the overall headcount, culture of the company (open plan or many enclosed offices), and the need for amenities eg. break out rooms, pantry, library and storage.
Size requirements vary on many factors, particularly how efficient the shape of the space is. It is best to err on the side of caution and avoid searching for space that could be too small. Perhaps allow for a buffer of 10% to be on the safe side.