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:

![enter image description here](https://i.stack.imgur.com/IHdcf.jpg)