public bool ScaleSymbols {get; set;}

var featureLayer = MapView.Active.Map.GetLayersAsFlattenedList().OfType<FeatureLayer>().FirstOrDefault();
ArcGIS.Desktop.Framework.Threading.Tasks.QueuedTask.Run(() =>
{
  // get the CIM layer definition
  var cimFeatureLayer = featureLayer.GetDefinition() as ArcGIS.Core.CIM.CIMFeatureLayer;
  // turn on the option to scale the symbols in this layer based in the map's reference scale
  cimFeatureLayer.ScaleSymbols = true;
});