el cuadro combinado utiliza un cuadro de texto para mostrar el contenido .. lo puede invalidar la plantilla y sólo tiene que añadir el "TextWrapping = Wrap" al cuadro de texto "PART_EditableTextbox":
<ControlTemplate x:Key="ComboBoxEditableTemplate" TargetType="{x:Type ComboBox}">
<Grid SnapsToDevicePixels="true">
<Border x:Name="Bd"
Background="{TemplateBinding Background}"
BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}"
Padding="1">
<Grid Grid.IsSharedSizeScope="true">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="1" />
<ColumnDefinition Width="*" />
<ColumnDefinition Width="Auto" SharedSizeGroup="ComboBoxButton" />
</Grid.ColumnDefinitions>
<TextBox x:Name="PART_EditableTextBox"
Grid.Column="1"
Margin="{TemplateBinding Padding}"
HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"
VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"
IsReadOnly="{Binding IsReadOnly, RelativeSource={RelativeSource TemplatedParent}}"
TextWrapping="Wrap"
Style="{StaticResource ComboBoxEditableTextBox}" />
<ToggleButton Grid.ColumnSpan="3"
Background="{x:Null}"
IsChecked="{Binding IsDropDownOpen, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}"
Style="{StaticResource ComboBoxTransparentButtonStyle}" />
</Grid>
</Border>
<Popup x:Name="PART_Popup"
AllowsTransparency="true"
Focusable="false"
IsOpen="{Binding IsDropDownOpen, RelativeSource={RelativeSource TemplatedParent}}"
Placement="Bottom"
PopupAnimation="{DynamicResource {x:Static SystemParameters.ComboBoxPopupAnimationKey}}">
<Microsoft_Windows_Themes:SystemDropShadowChrome x:Name="Shdw"
MinWidth="{TemplateBinding ActualWidth}"
MaxHeight="{TemplateBinding MaxDropDownHeight}"
Color="Transparent">
<Border x:Name="DropDownBorder"
Background="{DynamicResource {x:Static SystemColors.WindowBrushKey}}"
BorderBrush="{DynamicResource {x:Static SystemColors.WindowFrameBrushKey}}"
BorderThickness="1">
<ScrollViewer x:Name="DropDownScrollViewer">
<Grid RenderOptions.ClearTypeHint="Enabled">
<Canvas Width="0"
Height="0"
HorizontalAlignment="Left"
VerticalAlignment="Top">
<Rectangle x:Name="OpaqueRect"
Width="{Binding ActualWidth, ElementName=DropDownBorder}"
Height="{Binding ActualHeight, ElementName=DropDownBorder}"
Fill="{Binding Background, ElementName=DropDownBorder}" />
</Canvas>
<ItemsPresenter x:Name="ItemsPresenter"
KeyboardNavigation.DirectionalNavigation="Contained"
SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" />
</Grid>
</ScrollViewer>
</Border>
</Microsoft_Windows_Themes:SystemDropShadowChrome>
</Popup>
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="HasItems" Value="false">
<Setter TargetName="DropDownBorder" Property="MinHeight" Value="95" />
</Trigger>
<Trigger Property="IsEnabled" Value="false">
<Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.GrayTextBrushKey}}" />
<Setter TargetName="Bd" Property="Background" Value="{DynamicResource {x:Static SystemColors.ControlBrushKey}}" />
</Trigger>
<Trigger Property="IsGrouping" Value="true">
<Setter Property="ScrollViewer.CanContentScroll" Value="false" />
</Trigger>
<Trigger SourceName="PART_Popup" Property="HasDropShadow" Value="true">
<Setter TargetName="Shdw" Property="Margin" Value="0,0,5,5" />
<Setter TargetName="Shdw" Property="Color" Value="#71000000" />
</Trigger>
<Trigger SourceName="DropDownScrollViewer" Property="ScrollViewer.CanContentScroll" Value="false">
<Setter TargetName="OpaqueRect" Property="Canvas.Top" Value="{Binding VerticalOffset, ElementName=DropDownScrollViewer}" />
<Setter TargetName="OpaqueRect" Property="Canvas.Left" Value="{Binding HorizontalOffset, ElementName=DropDownScrollViewer}" />
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
Ahh, se También puede ser necesario configurar HorizontalContentAlign = Estirar y darle al control un ancho finito. –