You might find this useful. Try pasting this into a page using Kaxaml and playing around with the various parameters of the objects in the outer Grid. I find using Kaxaml for prototyping and experimenting with XAML layouts indispensable.
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto"/>
<ColumnDefinition Width="Auto"/>
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition/>
<RowDefinition/>
<RowDefinition/>
</Grid.RowDefinitions>
<!--
When I'm composing grids in XAML, I group things together by type, not by where
they live in the grid. This turns out to make a lot of maintenance tasks
easier.
Also, since Grid.Row and Grid.Column default to 0, a lot of people (and tools)
omit them if that's their value. Not me. It lets me quickly check to make
sure that content is where I think it is, just by looking at how it's organized
in the XAML.
-->
<TextBlock Grid.Row="0" Grid.Column="0" Grid.ColumnSpan="2" Background="Lavender" Padding="10" HorizontalAlignment="Stretch">Here's the first row of the outer grid.</TextBlock>
<TextBlock Grid.Row="2" Grid.Column="0" Grid.ColumnSpan="2" Background="Lavender" Padding="10" HorizontalAlignment="Stretch">Here's the third row of the outer grid.</TextBlock>
<TextBlock Grid.Row="1" Grid.Column="0" Background="AliceBlue" Padding="10">Here's the first column of the second row.</TextBlock>
<Grid Grid.Row="1" Grid.Column="1">
<Grid.ColumnDefinitions>
<!--
This part's pretty important. Setting up the SharedSizeGroups for these
two columns keeps the labels and text boxes neatly arranged irrespective of
their length.
-->
<ColumnDefinition SharedSizeGroup="Label"/>
<ColumnDefinition SharedSizeGroup="TextBox"/>
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition/>
<RowDefinition/>
<RowDefinition/>
</Grid.RowDefinitions>
<Label Grid.Row="0" Grid.Column="0">First label</Label>
<Label Grid.Row="1" Grid.Column="0">Second label</Label>
<Label Grid.Row="2" Grid.Column="0">Third label, containing unusually long content</Label>
<TextBox Grid.Row="0" Grid.Column="1">First text box, containing unusually long content</TextBox>
<TextBox Grid.Row="1" Grid.Column="1">Second text box</TextBox>
<TextBox Grid.Row="2" Grid.Column="1">Third text box</TextBox>
</Grid>
</Grid>