Editing any diagram shape does not take into account the fontsize of the shape.

A possible workaround is to wire to shape's Loaded event as below:
private void RadDiagramTextShape_Loaded(object sender, RoutedEventArgs e)
        {
            RadDiagramTextShape shape = sender as RadDiagramTextShape;
            TextBox tBox = shape.ChildrenOfType<TextBox>().FirstOrDefault();
            tBox.FontSize = shape.FontSize;
        }


Available in LIB version: 2016.3.926