Well i have not found or resolved why the code behind method does not work.. If someone discoveries the reason why that method fails to work Please post a resolution

In the meanwhile, I have use a client side scripting method using the ClientScript.js (as seen below)

function OnClientCommandExecuting(editor, args) {
    if (typeof jQuery === "undefined" || typeof jQuery.fn.dnnControlPanel === "undefined") return;
    
    var commandName = args.get_commandName();
    if (commandName == "ToggleScreenMode") {
        if (editor._isFullScreen == true ) {
            jQuery.fn.dnnControlPanel.show();
        }
        else {
            jQuery.fn.dnnControlPanel.hide();
        }
 }
 if (commandName == "ImageManager") {   

     var defin = editor.get_dialogOpener()._getDialogDefinition(commandName);
     // now we can set the width and/or height of the Manager which in turn sets the RadWindow Wrapper
     defin.Width = '900px';

 }


}

Cheers,