H
Orientation="Horizontal". If you do that, then items will be stacked horizontally. That's what the stackpanel is for: stacking. If you need to align stuff on different side, you'll need dockpanel, as you already figured out
VHorizontalAlignment of the first TextBlock to Right, both would occupy the same space. DockPanel.Dock="Right" on both TextBlocks, the first one will dock to the wall and take as much space as it needs then the second one will dock next to that one and take as much space as it needs
DOrientation="Horizontal"HorizontalAlignmentDockPanel.Dock="Right"<DockPanel Grid.Column="0" Grid.Row="0">
<TextBlock Text="File size:"/>
<TextBlock Text="0.00 MB" DockPanel.Dock="Right"/>
</DockPanel>