The average class size at McGill in first and second years (overall, not just Arts) appears to be 65.5, according to the Maclean’s rankings. However, that really depends on the program (I’m sure political science introductory courses are large, and economics, which is probably more populated than, say, English, also overlaps with different faculties, including Arts). However, you can also enroll in first-year seminars with a maximum number of 25 students: