We have an acre that is double chain link fenced against coyote intrusions... Although at about 70 or so pounds, Serena is on the small side for a Great Pyrenees; she has been doing her guardian duties quite effectively. Since we adopted Serene haven't noticed the lone coyote who had been frequently surveying our property before Serena joined our family.
She knows that this property is hers and also knows who is allowed to enter it. No problems with the gardener or the pool maintenance guy but. anyone else needs to face a barking Serena (until we introduce Serena to the intruder). She doesn't bite but, will make it obvious that the intruder is not welcome.