Regardless of how long the law says a chain should be, this isnt a bike or car we're talking about - its a dog, clearly in distress, and clearly mistreated. It shouldn't be chained at all
Call the (whatever your equivalent of the) RSPCA (is) and they'll decide whether its being mistreated or not. Sometimes they wont accept anonymous calls - they didn't when i called, but they didn't disclose my details (it was someone I knew). If they take it to a shelter, you can always give them your details and arrange something yourself if they can't find a home for him.