When I first started in this area on upland birds, to go with a recently acquired English double, an old mentor of mine in that field, gave me a few boxes of C.I.L. roll crimped paper case # 7 shells which I foolishly used on Ruff Grouse locally.
In reality, for Grouse, I don't think there's much difference to decide on 6's or 7 1/2's.