How many members of Business have the Medium size of tee shirt?
members of Economics refers to major_name = 'Business'; t_shirt_size = 'Medium'
SELECT COUNT(T2.member_id) FROM major AS T1 INNER JOIN member AS T2 ON T1.major_id = T2.link_to_major WHERE T1.major_name = 'Business' AND T2.t_shirt_size = 'Medium'