How can I Make Variable Size of Tiles in win8 metro style

<common:LayoutAwarePage
    x:Name="pageRoot"
    x:Class="FairyTales.GroupedItemsPage"
    DataContext="{Binding DefaultViewModel, RelativeSource={RelativeSource Self}}"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:FairyTales"
    xmlns:data="using:FairyTales.Data"
    xmlns:common="using:FairyTales.Common"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d">
 
    <Page.Resources>
 
        <!--
            Collection of grouped items displayed by this page, bound to a subset
            of the complete item list because items in groups cannot be virtualized
        -->
        <CollectionViewSource
            x:Name="groupedItemsViewSource"
            Source="{Binding Groups}"
            IsSourceGrouped="true"
            ItemsPath="TopItems"
            d:Source="{Binding AllGroups, Source={d:DesignInstance Type=data:SampleDataSource, IsDesignTimeCreatable=True}}"/>
    </Page.Resources>
 
    <!--
        This grid acts as a root panel for the page that defines two rows:
        * Row 0 contains the back button and page title
        * Row 1 contains the rest of the page layout
    -->
    <Grid Style="{StaticResource LayoutRootStyle}">
        <Grid.RowDefinitions>
            <RowDefinition Height="140"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>
 
        <!-- Horizontal scrolling grid used in most view states -->
        <GridView
            x:Name="itemGridView"
            AutomationProperties.AutomationId="ItemGridView"
            AutomationProperties.Name="Grouped Items"
            Grid.RowSpan="2"
            Padding="116,137,40,46"
            ItemsSource="{Binding Source={StaticResource groupedItemsViewSource}}"
            ItemTemplate="{StaticResource Standard250x250ItemTemplate}"
            SelectionMode="None"
            IsSwipeEnabled="false"
            IsItemClickEnabled="True"
            ItemClick="ItemView_ItemClick"   Loaded="itemGridView_Loaded" >
        	<GridView.Background>
        		<ImageBrush ImageSource="Assets/bck.png"/>
        	</GridView.Background>
 
            <GridView.ItemsPanel>
                <ItemsPanelTemplate>
                    <VirtualizingStackPanel Orientation="Horizontal"/>
                </ItemsPanelTemplate>
            </GridView.ItemsPanel>
            <GridView.GroupStyle>
                <GroupStyle>
                    <GroupStyle.HeaderTemplate>
                        <DataTemplate>
                            <Grid Margin="1,0,0,6">
                                <Button
                                    AutomationProperties.Name="Group Title"
                                    Click="Header_Click"
                                    Style="{StaticResource TextPrimaryButtonStyle}" >
                                    <StackPanel Orientation="Horizontal">
                                        <TextBlock Text="{Binding Title}" Margin="3,-7,10,10" Style="{StaticResource GroupHeaderTextStyle}" />
                                        <TextBlock Text="{StaticResource ChevronGlyph}" FontFamily="Segoe UI Symbol" Margin="0,-7,0,10" Style="{StaticResource GroupHeaderTextStyle}"/>
                                    </StackPanel>
                                </Button>
                            </Grid>
                        </DataTemplate>
                    </GroupStyle.HeaderTemplate>
                    <GroupStyle.Panel>
                        <ItemsPanelTemplate>
                            <VariableSizedWrapGrid Orientation="Vertical" Margin="0,0,80,0"/>
                        </ItemsPanelTemplate>
                    </GroupStyle.Panel>
                </GroupStyle>
            </GridView.GroupStyle>
        </GridView>
 
        <!-- Vertical scrolling list only used when snapped -->
        <ListView
            x:Name="itemListView"
            AutomationProperties.AutomationId="ItemListView"
            AutomationProperties.Name="Grouped Items"
            Grid.Row="1"
            Visibility="Collapsed"
            Margin="0,-10,0,0"
            Padding="10,0,0,60"
            ItemsSource="{Binding Source={StaticResource groupedItemsViewSource}}"
            ItemTemplate="{StaticResource Standard80ItemTemplate}"
            SelectionMode="None"
            IsSwipeEnabled="false"
            IsItemClickEnabled="True"
            ItemClick="ItemView_ItemClick">
 
            <ListView.GroupStyle>
                <GroupStyle>
                    <GroupStyle.HeaderTemplate>
                        <DataTemplate>
                            <Grid Margin="7,7,0,0">
                                <Button
                                    AutomationProperties.Name="Group Title"
                                    Click="Header_Click"
                                    Style="{StaticResource TextPrimaryButtonStyle}">
                                    <StackPanel Orientation="Horizontal">
                                        <TextBlock Text="{Binding Title}" Margin="3,-7,10,10" Style="{StaticResource GroupHeaderTextStyle}" />
                                        <TextBlock Text="{StaticResource ChevronGlyph}" FontFamily="Segoe UI Symbol" Margin="0,-7,0,10" Style="{StaticResource GroupHeaderTextStyle}"/>
                                    </StackPanel>
                                </Button>
                            </Grid>
                        </DataTemplate>
                    </GroupStyle.HeaderTemplate>
                </GroupStyle>
            </ListView.GroupStyle>
        </ListView>
 
        <!-- Back button and page title -->
        <Grid>
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="Auto"/>
                <ColumnDefinition Width="*"/>
            </Grid.ColumnDefinitions>
            <Button x:Name="backButton" Click="GoBack" IsEnabled="{Binding Frame.CanGoBack, ElementName=pageRoot}" Style="{StaticResource BackButtonStyle}"/>
            <TextBlock x:Name="pageTitle" Text="{StaticResource AppName}" Grid.Column="1" IsHitTestVisible="false" Style="{StaticResource PageHeaderTextStyle}"/>
            <ProgressBar Name="prgbr" HorizontalAlignment="Left" Height="200" VerticalAlignment="Top" Width="1412" Grid.ColumnSpan="2" IsIndeterminate="True" Foreground="#FFE4089B" FontWeight="Bold"/>
        </Grid>
 
        <VisualStateManager.VisualStateGroups>
 
            <!-- Visual states reflect the application's view state -->
            <VisualStateGroup x:Name="ApplicationViewStates">
                <VisualState x:Name="FullScreenLandscape"/>
                <VisualState x:Name="Filled"/>
 
                <!-- The entire page respects the narrower 100-pixel margin convention for portrait -->
                <VisualState x:Name="FullScreenPortrait">
                    <Storyboard>
                        <ObjectAnimationUsingKeyFrames Storyboard.TargetName="backButton" Storyboard.TargetProperty="Style">
                            <DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource PortraitBackButtonStyle}"/>
                        </ObjectAnimationUsingKeyFrames>
 
                        <ObjectAnimationUsingKeyFrames Storyboard.TargetName="itemGridView" Storyboard.TargetProperty="Padding">
                            <DiscreteObjectKeyFrame KeyTime="0" Value="96,137,10,56"/>
                        </ObjectAnimationUsingKeyFrames>
                    </Storyboard>
                </VisualState>
 
                <!--
                    The back button and title have different styles when snapped, and the list representation is substituted
                    for the grid displayed in all other view states
                -->
                <VisualState x:Name="Snapped">
                    <Storyboard>
                        <ObjectAnimationUsingKeyFrames Storyboard.TargetName="backButton" Storyboard.TargetProperty="Style">
                            <DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource SnappedBackButtonStyle}"/>
                        </ObjectAnimationUsingKeyFrames>
                        <ObjectAnimationUsingKeyFrames Storyboard.TargetName="pageTitle" Storyboard.TargetProperty="Style">
                            <DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource SnappedPageHeaderTextStyle}"/>
                        </ObjectAnimationUsingKeyFrames>
 
                        <ObjectAnimationUsingKeyFrames Storyboard.TargetName="itemListView" Storyboard.TargetProperty="Visibility">
                            <DiscreteObjectKeyFrame KeyTime="0" Value="Visible"/>
                        </ObjectAnimationUsingKeyFrames>
                        <ObjectAnimationUsingKeyFrames Storyboard.TargetName="itemGridView" Storyboard.TargetProperty="Visibility">
                            <DiscreteObjectKeyFrame KeyTime="0" Value="Collapsed"/>
                        </ObjectAnimationUsingKeyFrames>
                    </Storyboard>
                </VisualState>
            </VisualStateGroup>
        </VisualStateManager.VisualStateGroups>
    </Grid>
</common:LayoutAwarePage>