Is there some size-threshold where the service/therapy animal must have its own seat? Larger than will fit in a duffel bag at your feet? Encroaching on neighbo

Is there some size-threshold where the service/therapy animal must have its own seat? Larger than will fit in a duffel bag at your feet? Encroaching on neighboring passenger's legroom? The article does say "airlines are still allowed to reject certain approved animals if they're deemed too large, too dirty, or too dangerous"