I believe that FontFamily is PhoneFontFamilySemiBold , and FontSize is PhoneFontSizeNormal .
Below is a snippet of code from the HubTile , where you can pretty much find all the information about tile style. :)
<TextBlock x:Name="BackTitleBlock" Grid.Row="1" VerticalAlignment="Bottom" Margin="10,0,0,6" FontFamily="{StaticResource PhoneFontFamilySemiBold}" FontSize="{StaticResource PhoneFontSizeNormal}" Foreground="{TemplateBinding Foreground}" TextWrapping="NoWrap"/>
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