如何在按钮模板触发器中使用内容大小?

我正在为一个按钮创建一个ControlTemplate。我想动画的大小,使其增长,以适应内容时,鼠标悬停在它。但是,我不确定如何获得触发器部分的内容大小。这是我的东西。

<ControlTemplate x:Key="buttonTemplate" TargetType="Button">
<Grid Name="grid" Height="12" Width="12" Opacity="0.4">
<ContentPresenter Name="content" ... />
</Grid>

<ControlTemplate.Triggers>
    <EventTrigger RoutedEvent="MouseEnter">
        <BeginStoryboard>
            <Storyboard>
        <DoubleAnimation
            Storyboard.TargetName="grid"
            Storyboard.TargetProperty="Width"
            To="40"                         
            Duration="0:0:0.4"/>    
        <DoubleAnimation
            Storyboard.TargetName="grid"
                Storyboard.TargetProperty="Height"
            To="20"                         
            Duration="0:0:0.4"/>    
        </Storyboard>
    </BeginStoryboard>
    </EventTrigger>

    <EventTrigger RoutedEvent="MouseLeave">
    <BeginStoryboard>
        <Storyboard>
            <DoubleAnimation
            Storyboard.TargetName="grid"
            Storyboard.TargetProperty="Width"
            Duration="0:0:0.2"/>    
        <DoubleAnimation
            Storyboard.TargetName="grid"
            Storyboard.TargetProperty="Height"
            Duration="0:0:0.2"/>                        
        </Storyboard>
    </BeginStoryboard>
    </EventTrigger>

</ControlTemplate.Triggers>
</ControlTemplate>

TemplateBinding To="40" 但这会导致异常。如何在触发器中使用动态值?