<ScrollViewer Grid.Row="2">

    <Grid>

        <Grid.RowDefinitions>

        <RowDefinition Height="Auto"/>

    </Grid.RowDefinitions>

        <Grid Grid.Row="0" HorizontalAlignment="Left" Margin="10,5,0,0" Width="500">

            <Grid.RowDefinitions>

                <RowDefinition Height="Auto" />

                <RowDefinition Height="Auto" />

            </Grid.RowDefinitions>

            <Grid Grid.Row="0">

                <Grid.Background >

                    <ImageBrush ImageSource="Public\Images\chat_green-textarea.png"/>

                </Grid.Background>

                <TextBlock Padding="5" Foreground="White" FontSize="15" TextWrapping="Wrap">Lorem Ipsum is simply dummy text of the printing and typesetting industry.

 </TextBlock>                                    

            </Grid>

            <Grid Grid.Row ="1">

                <Image Source="Public\Images\chat_green-textarea-tail.png" Height="20" Width="30" HorizontalAlignment="Left" Margin="10,-4,0,0"/>

            </Grid>

        </Grid>

    </Grid>

</ScrollViewer>

I am displaying a Text in
a Grid, but I want its max width
to be set equals to the 75% of its parent grid. Thanks!