Don't overestimate how many people can "comfortably" jamming in that space. In a typical classroom setup, if the room was originally intended for 60 students in full capacity, then for game jam purposes, it's better to cut that number in half, say 30. The jammers will be there for not only 2~3 hours, but 48 hours. Participants need a lot more space to put their belongings, equipments, standing up relaxing... etc.