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>