I think in Stackpanel instead of setting Height you should set MaxHeight and VerticalAlignment property value should be Center instead of Bottom and that should do your work:
<DataTemplate x:Key="TileViewDT">
<DockPanel>
<StackPanel MaxHeight="40.5" DockPanel.Dock="Right" Margin="5,2,5,2" VerticalAlignment="Center">
<TextBlock x:Name="Name" TextWrapping="Wrap" Text="{Binding XPath=@Name}" Width="132" />
<TextBlock x:Name="Size" Foreground="DarkGray" TextWrapping="Wrap" Text="{Binding XPath=@Size}" />
</StackPanel>
<Image x:Name="Img" Source="BtnImg/Computer.png" Stretch="Fill" Margin="10,0,5,0" Width="48" Height="48"/>
</DockPanel>
</DataTemplate>
Hope it works !!!