Name of this element

OK, I've fixed that so thanks.

        <GridView Grid.Row="1">
            <ScrollViewer>
                <StackPanel Orientation="Horizontal">
                    <!-- group starts here -->
                    <StackPanel>
                        <TextBlock Text="Group x"/>
                        <StackPanel>
                            <!-- put text blocks here.  -->                            
                        </StackPanel>
                    </StackPanel>
                    <!-- group ends right above this line -->

                    <!-- group starts here -->
                    <StackPanel>
                        <TextBlock Text="Group x"/>
                        <StackPanel>
                            <!-- put text blocks here.  -->
                        </StackPanel>
                    </StackPanel>
                    <!-- group ends right above this line -->
                </StackPanel>
            </ScrollViewer>
        </GridView>

1) Out of interest based to the image above what is the default font size used for 'Group Title: 1>'?

2) Also in Windows Phone there are 2 text styles I regularly use: PhoneTextLargeStyle (font size 32) and PhoneTextSubtleStyle (font size 20). Is it a good idea to use those font sizes to get the same sort of appearance in my Windows Store app?

3) What is the default width & height (in pixels used) for groups?