By applying a Style on the X Axis label TextBlock you can control various properties such as Width, Font, FontSize and so on. Here you can find information exactly how to do that.
You can use the radChart.DefaultView.ChartArea.AxisX.DefaultLabelFormat string to specify the exact format of the X Axis labels. For more informations on the format expressions supported by the RadChart you can check this help topic.
And if you want to hide the X axis labels altogether you can use the radChart.DefaultView.ChartArea. AxisX.AxisLabelsVisibility property.