For a spruce graft, it's almost certainly just a cosmetic issue, because nearly all spruce are grafted onto Picea abies or pungens, so there isn't the chance that the grafter tried to use something incompatible...

The scion does not have to be the same size as your rootstock, as long as the cambium layer is matched on at least one side. For low graft, unions that are not going to be visible, it really doesn't matter. However, I can't imagine grafting a scion on standard that didn't have the same diameter as the rootstock.. I would want them to match in that case, since the union is going to be highly visible. There is no reason it should fail, but I might pass on it just because the union probably will never look great, and it doesn't speak well of the professionalism of the grafter/wholesaler that supplied it...