public bool ScaleSymbols {get; set;}

{
    // get the CIM layer definition
    // Note: needs to be called on the MCT
    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;
}