Bone marrow. The inside of your bones are filled with a soft tissue called marrow. There are two types of bone marrow: red and yellow. Red bone marrow is where all new red blood cells, white blood cells, and platelets are made.
Mine is 8m x 7m (3m is 10 feet, so around 27 x 23 feet). This is normal for an Australian classroom, although there's a fair bit of variation.
Classroom Size: Classrooms are usually too small, but a classroom that is too large can be problematic also. If the classroom is too large, there are certain strategies to account for this, but one that is too small puts students on top of one another and really makes everyone uncomfortable.