According to the design guidelines, the width of a ContextualTab is determined by the sum of the widths of the RibbonTabs. However, there are cases where the width of the containing tabs is less than the width needed by the ContextualTab to display its text. To handle this case, we should allow the users to set the width of the ContextualTabs.

WORKAROUND: increase the size of the inner tabs:
 private void radButton1_Click(object sender, EventArgs e)
        {
            ribbonTab1.MinSize = new Size(ribbonTab1.Size.Width+60, 0);
            radRibbonBar1.RibbonBarElement.InvalidateMeasure(true);
            radRibbonBar1.RibbonBarElement.UpdateLayout();
            radRibbonBar1.RibbonBarElement.RibbonCaption.CaptionLayout.InvalidateMeasure(true);
            radRibbonBar1.RibbonBarElement.RibbonCaption.CaptionLayout.UpdateLayout();
        }