The creation of a molecular-scale covalently bound assembly of fullerene C(60) molecules has been precisely controlled in ultrathin multilayer films of C(60) molecules. When a negative sample bias voltage is applied to a tunneling junction between the C(60) film and the tip of a scanning tunneling microscope (STM), a C(60) molecule beneath the tip covalently bonds to an adjacent molecule in the underneath layer. We show that such a chemical reaction is not necessarily limited to the top and second layers of the C(60) film and that the resulting C(60) oligomer can be tuned to form a dimer, trimer, tetramer, or pentamer; the number of interconnected C(60) molecules increases one by one upon increasing the magnitude of the local electric field under the STM tip. The created oligomers are linear chains of C(60) molecules starting from the top layer and aligned toward the interface layer in the multilayer C(60) films. We consider that the electrostatic negative ionization of C(60) molecules and its spatial distribution in the multilayer C(60) film are critical factors in achieving size-tunable oligomerization.