If you are talking about the plug which is in line with the front two freeze plugs.... mine has a flat to flat measurement of .730 inches... which , when I compared to my metric wrenches seemed to be exactly 18 mm.... I don't have large allens With the size marked on them ....
If you have the allen MB oil pan drain plug wrench.... it would not surprise me if they thought ahead enough to have this the same size as that one.... good luck...