The standard field box or its equivalent, when used as a unit of trade or for reporting purposes, shall be of the uniform standard size of 311/2 inches long, 13 inches high, and 12 inches wide, inside measurements, and shall be divided into two compartments by a center partition of at least three-fourths inch thickness, and each of these compartments thus created shall have a cubical capacity that does not exceed 2,400 cubic inches.