The current workaround is to edit the control template of the control and set the MinWidth of the PART_ListBox element:
<Popup x:Name="PART_Popup">
<Grid x:Name="PopupRoot">
<telerik:RadListBox x:Name="PART_ListBox"
MinWidth="{TemplateBinding MinDropDownWidth}"
MaxHeight="{TemplateBinding MaxDropDownHeight}"
ItemsSource="{TemplateBinding FilteredItems}"
ItemTemplate="{TemplateBinding DropDownItemTemplate}"
FontFamily="{TemplateBinding FontFamily}"
FontSize="{TemplateBinding FontSize}"/>
<Border
Background="{Binding Background, ElementName=PART_ListBox}"
BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="1"
Width="{Binding ElementName=PART_ListBox, Path=Width}"
Visibility="{Binding ElementName=PART_NoResultsContentPresenter, Path=Visibility}">
<ContentPresenter x:Name="PART_NoResultsContentPresenter"
MinWidth="{TemplateBinding MinDropDownWidth}"
MaxHeight="{TemplateBinding MaxDropDownHeight}"
Content="{TemplateBinding NoResultsContent}"
ContentTemplate="{TemplateBinding NoResultsContentTemplate}"
Visibility="Collapsed"
Margin="5 4"/>
</Border>
</Grid>
</Popup>