Abstract: An apparatus for controlling the size of a wafer supporting boat such that the wafers maintained by the boat have to be positioned at predetermined distances from at least two specific utmost faces of said boat. This apparatus comprises a slanted plane (31), two reference surfaces (32), guiding means (34) arranged in order that, when the boat is laid on the slanted plane, it may slide on it in such a way that the utmost faces abut against the reference surfaces, at least one plate (50) provided with a portion of reference groove (51) arranged so as to prolong according to their plane the supporting grooves of a wafer at a determined position. Thus, when the utmost faces are abuting against the reference surfaces, a wafer can be inserted into its supporting grooves and into the reference groove only if the size of the boat corresponds to a determined standard.