A couple of Hershey's Kisses, a rib or two of a milk chocolate bar, or a Reese's PBC aren't going to kill a medium to large-sized dog, nor will a one-pound box of chocolate fudge (my parents had a large Lab who stole one and ate it, with no ill effects). It's best to keep that kind of thing out of reach, because most dogs love chocolate. I'd be concerned about a Chihuahua or Yorkie getting too much, but there's no reason to panic if your Shepherd or Dane steals a chocolate bar.