  The typical material bismuth shows a several-fold change in electrical
resistivity at 4 K over a 400 {\AA} film thickness period. In 2021, we
discovered a novel size effect with a period of 25 {\AA} on strongly correlated
compound CaRuO_3. The change in film thickness of only one nanometer leads to
an increase in electrical resistivity at 4 K by a factor of several billion.
However, the excitation energy of 2.4 eV on insulating CaRuO_3 is too large to
explain the mechanism by conventional quantum well. In this study, we clarify
that the increases in electrical resistivity are accompanied by lattice
expansion and caused by the Mott transitions. We measured in-plane X-ray
diffraction on CaRuO_3 films and found a 24.8 {\AA} periodic thickness
oscillation of the lattice spacing d_{(004)}. We determined the nesting vector
from the Fermi surface and revealed the spin density wave with a period
equivalent to the size effect. We concluded that the antiferromagnetic
correlation appearing in the boundary conditions triggers the periodic Mott
insulators depending on the film thickness. We refer to the remarkable size
effect stemming from strong electron correlation as the 'Mott size effect'.
