selecting listbox item, e.g. fontsize?

How can I select an item in a select box and have it take effect?

I tried all the function I could find, but none worked. The value is selected in the list, but does not take effect.

Using FCKeditor 2.6.1, when using 2.6.4 or later I believe you need to use 'Combo' instead of '_Combo'

editorInstance.EditorWindow.parent.FCKToolbarItems.GetItem('FontSize')._Combo.SelectItemByLabel('18pt');
editorInstance.ToolbarSet.CurrentInstance.Commands.GetCommand('FontSize').Execute('18pt', editorInstance.EditorWindow.parent.FCKToolbarItems.GetItem('FontSize')._Combo.Items['18pt']) ;
editorInstance.EditorWindow.parent.FCKToolbarItems.GetItem('FontSize')._Combo.OnSelect('18pt', editorInstance.EditorWindow.parent.FCKToolbarItems.GetItem('FontSize')._Combo.Items['18pt']);
editorInstance.EditorWindow.parent.FCKToolbarItems.GetItem('FontSize')._Combo.SetLabel('18pt');
editorInstance.EditorWindow.parent.FCKToolbarItems.GetItem('FontSize')._Combo.SelectItem('18pt');

By the way _Combo._OuterTable is the DOM node of the field-like element visible in the toolbar and _Combo._Panel.MainNode is the panel (the list of items) that becomes visible when clicking on the toolbar element (again, using FCKeditor 2.6.1)