The Courant number is based on the cell volume, but not via the cubic root. In the expression: Co = U * dt / delta, the ratio of the velocity and length scales, U / delta, is approximated as the mass flux coming into the cell, divided by the density and the cell volume. For example, in a simplified case of a a Cartesian mesh with a flow along the X-axis, the Courant number becomes, U_x * dt / dx, because the mass flux = rho * U_x * dy * dz.