<Button xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Height="70" Width="100">
        <Button.Triggers>
        <EventTrigger RoutedEvent="Button.MouseEnter">
            <BeginStoryboard>
                <Storyboard>
                    <DoubleAnimation
             Storyboard.TargetProperty="Width"
                 Duration="0:0:4.5"
                 From="100" To="400"/>
                </Storyboard>
            </BeginStoryboard>
        </EventTrigger>
        <EventTrigger RoutedEvent="Button.MouseLeave">
            <BeginStoryboard>
                <Storyboard>
                    <DoubleAnimation
             Storyboard.TargetProperty="Width"
                 Duration="0:0:4.5"
                 From="400" To="100"/>
                </Storyboard>
            </BeginStoryboard>
        </EventTrigger>
    </Button.Triggers>
        OK
</Button>