Can't stretch a control in a stackpanel - silverlight

Question

Here is a piece of my control code:
<telerik:RadWindow>
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="20"/>
<RowDefinition Height="20"/>
<RowDefinition Height="*"/>
<RowDefinition Height="40"/>
</Grid.RowDefinitions>
<StackPanel Grid.Row="2" HorizontalAlignment="Stretch" Background="Blue">
<Grid
x:Name="DetailsGrid">
<prismvm:DataTemplateSelector
Content="{Binding NewTemplate}">
<prismvm:DataTemplateSelector.Resources>
<DataTemplate
x:Key="EmailMessageTemplate">
<StackPanel Orientation="Vertical" HorizontalAlignment="Stretch" Background="Red">
<TextBlock Text="Name: "/>
<TextBox Text="{Binding Name,Mode=TwoWay}" HorizontalAlignment="Stretch" Background="Green"/>
<TextBlock Text="Subject: "/>
<TextBox Text="{Binding EmailSubject,Mode=TwoWay}" HorizontalAlignment="Stretch"/>
<TextBlock Text="Body: "/>
<TextBox Text="{Binding Body,Mode=TwoWay}" TextWrapping="Wrap"/>
</StackPanel>
</DataTemplate>
<DataTemplate
x:Key="SmsMessageTemplate">
<StackPanel Orientation="Vertical">
<TextBlock Text="Subject: "/>
<TextBox Text="{Binding Name,Mode=TwoWay}"/>
<TextBlock Text="Sms body: "/>
<TextBox Text="{Binding Body,Mode=TwoWay}"/>
</StackPanel>
</DataTemplate>
</prismvm:DataTemplateSelector.Resources>
</prismvm:DataTemplateSelector>
</Grid>
</StackPanel>
</telerik:RadWindow>
The problem is that at start the width of the textboxes is very little and I need to have width the same as grid.
How can I do it?
EDIT
I've updated my xaml, and now every element looks like at the picture:

Answer 1

I've added
HorizontalContentAlignment="Stretch"
to
<prismvm:DataTemplateSelector
Content="{Binding NewTemplate}">
and now it works fine.

Answer 2

A StackPanel does not stretch to fit its parent container by default (in this case column 0 of the Grid, or rather the entire grid).
Try this on the outer StackPanel:
<StackPanel Grid.Row="2" HorizontalAlignment="Stretch">
Then the child TextBoxes have something to stretch to.
When in doubt, set different background colours on your various containers so you can see how much room they take up at runtime.