K-Point grid choise:
8x8x1; as done for H2 on Cu(111) by Mondal and Wijzenbroek et. al., 2013, J. Phys. Chem. 



Quick and Dirty Slab Info:

Lattice Constant is: 3.697593061159407; via HSE06-1/2x-vdW-DF2 Lattice relaxation (see Lattice constant part of data)

Interlayer spacing, d23 and higher seems not to change from bulk (see lit ref)
Thus only d12 needs to change. In previous work: Mondal, Wijzenbroek et. al. JPC 2013 -1.3% is used.
However, the reference they use (see lit ref below) has no 0 K interlayer spacing and would at the very least hint at a value of -1.0%

d12 = -1.0% at RT
Lit Ref:
K. H. Chae, H.C. Lu and T. Gustafsson, 1996, Pys. Rev. B. 54. 19.