Put your stuff into a grid, bind the grid's scale render transformation to a slider (slider should have min value of 1):
<Grid x:Name="LayoutRoot">
<Grid.RowDefinitions>
<RowDefinition Height="0.867*"/>
<RowDefinition Height="0.133*"/>
</Grid.RowDefinitions>
<Slider x:Name="slider" Grid.Row="1" Minimum="1"/>
<Grid RenderTransformOrigin="0.5,0.5">
<Grid.RenderTransform>
<TransformGroup>
<ScaleTransform
ScaleY="{Binding Path=Value, ElementName=slider}"
ScaleX="{Binding Path=Value, ElementName=slider}"/>
</TransformGroup>
</Grid.RenderTransform>
<TextBox Text="TextBox" Height="45.214"
VerticalAlignment="Top" Margin="194,139,209,0"/>
<TextBox VerticalAlignment="Bottom"
Text="TextBox" Margin="194,0,209,118.254" Height="48.96"/>
</Grid>
</Grid>
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…