I have 55 merge fields right now in the RadRichTextBox Merge collection. As you have implemented it, this list does not have a scroll bar when the list gets cut off at the bottom of the screen. Here is my current XAML, which is from your template:
<telerik:RadRibbonDropDownButton Name="MailingsInsertFieldButton" CollapseToMedium="WhenGroupIsMedium" telerik:ScreenTip.Description="You can insert fields such as Name or Address, which will be replaced automatically with information from a database or contact list for each copy of the form letter." LargeImage="pack://application:,,,/Telerik.Windows.Controls.RichTextBoxUI;component/Images/MSOffice/32/InsertMergeField.png" telerik:RadRichTextBoxRibbonUI.RichTextCommand="{Binding InsertMergeFieldEmptyCommand}" Size="Large" Text="Insert Merge Field" telerik:ScreenTip.Title="Insert Merge Field" />