<YourControl.Triggers>

     <EventTrigger RoutedEvent="YourControl.Loaded"><!--这里是关键-->

         <BeginStoryboard>

           <Storyboard>

             <DoubleAnimation Storyboard.TargetProperty="Width" From="" To="" Duration="0:0:5" />

             <DoubleAnimation Storyboard.TargetProperty="Height" From="" To="" Duration="0:0:5" />

           </Storyboard>

         </BeginStoryboard>

     </EventTrigger>

 </YourControl.Triggers>