It is any loose rock that is larger than 2 mm ( 0.079 in ) in its smallest dimension ( about 1\/12 of an inch ) and no more than 64 mm ( 2.5 in ) . Specifically , it is any loose rock that is larger than 2 mm ( 0.079 in ) in its smallest dimension ( about 1\/12 of an inch ) and no more than 64 m. The next smaller size class in geology is sand , which is ) 0.0625 t in size .