5 times longer.
In the typical right-handed Cartesian coordinate system, the (210) face runs parallel to the z-axis, cuts the y-axis at the cell edge and cuts the x-axis half-way along the cell edge. Don't forget that the Miller indices are reciprocals so that 2 means 1/2. Shown here is such a surface shaded in light green.
The surface atom arrangement becomes more complex on this surface than on the (100) or (111) surfaces. The surface is more open, revealing several layers of subsurface atoms. This consequently introduces a corrugation into the surface. This will affect the reactivity of the surface as it introduces a large concentration of atoms lying along step edges where their coordination is somewhat unsatisfied. A top view and a side view of this surface is shown here; the darker coloured atoms lie lower than the lighter ones. Note that the surface lattice is rectangular. The atoms lying in the rows have the same spacing as is found on the (100) surface, but the distance perpendicular to the row direction is 5 times longer.