By default RadComboBox behavior is that it will show as many items as possible, based on the browser's viewport. For showing the remaining items one scroll bar will appear inside the dropdownlist of RadComboBox. Please have a look into the sample code snippet which works fine at my end and also take a look into the attached screen shot.
sender, EventArgs e)
index = 0; index < 150; index++)
Telerik.Web.UI.RadComboBoxItem item =
item.Text = index.ToString();
Let me know if you have any concern.