public bool UsesRealWorldSymbolSizes {get;}

{
    //Note: Needs QueuedTask to run
    //Get simple renderer from the feature layer
    CIMSimpleRenderer currentRenderer = featureLayer.GetRenderer() as CIMSimpleRenderer;
    if (currentRenderer == null)
        return;

    //Set symbol's real world setting to be the same as that of the feature layer
    symbol.SetRealWorldUnits(featureLayer.UsesRealWorldSymbolSizes);

    //Update the symbol of the current simple renderer
    currentRenderer.Symbol = symbol.MakeSymbolReference();
    //Update the feature layer renderer
    featureLayer.SetRenderer(currentRenderer);
}

{
    //Note: Needs QueuedTask to run
    //Get simple renderer from the feature layer
    CIMSimpleRenderer currentRenderer = featureLayer.GetRenderer() as CIMSimpleRenderer;
    if (currentRenderer == null)
        return;
    //Get symbol from the SymbolStyleItem
    CIMSymbol symbolToApply = symbolStyleItem.Symbol;

    //Set symbol's real world setting to be the same as that of the feature layer
    symbolToApply.SetRealWorldUnits(featureLayer.UsesRealWorldSymbolSizes);

    //Update the symbol of the current simple renderer
    currentRenderer.Symbol = symbolToApply.MakeSymbolReference();
    //Update the feature layer renderer
    featureLayer.SetRenderer(currentRenderer);
}