ScaleTransform

<!-- Zoom x2 sur le TextBlock quand la souris passe dessus -->
<TextBlock Text="Texte"
           RenderTransformOrigin="0.5 0.5"> <!-- RenderTransformOrigin permet de centrer le zoom -->
    <TextBlock.Style>
        <Style TargetType="{x:Type TextBlock}">
            <Style.Triggers>
                <Trigger Property="IsMouseOver"
                         Value="True">
                    <Setter Property="RenderTransform">
                        <Setter.Value>
                            <ScaleTransform ScaleX="2"
                                            ScaleY="2" />
                        </Setter.Value>
                    </Setter>
                </Trigger>
            </Style.Triggers>
        </Style>
    </TextBlock.Style>
</TextBlock>