Animate Width and Height of a Rectangle

   
     

<Window x:Class="Main"
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  Title="" Height="300" Width="300">
  <Window.Resources>
    <Storyboard x:Key="rect1Storyboard" Storyboard.TargetName="rect1">
      <ParallelTimeline>
        <DoubleAnimation To="50" Duration="0:0:10" FillBehavior="Stop" Storyboard.TargetProperty="Width" />
        <DoubleAnimation To="50" Duration="0:0:5" FillBehavior="HoldEnd" AccelerationRatio="0.5" DecelerationRatio="0.25" Storyboard.TargetProperty="Height" />
      </ParallelTimeline>
    </Storyboard>
  </Window.Resources>
  <Window.Triggers>
    <EventTrigger RoutedEvent="Rectangle.Loaded" SourceName="rect1">
      <BeginStoryboard Storyboard="{StaticResource rect1Storyboard}" />
    </EventTrigger>
  </Window.Triggers>
  <Grid>
    <Rectangle x:Name="rect1" Margin="10" Width="100" Height="100" Fill="Firebrick" Grid.Column="1" />
  </Grid>
</Window>