Your StackPanel
is not in Grid
, it`s inside Border
. So for it to take all available space you can set horizontal and vertical alignment to Stretch
both for it and its parent Border
:
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="1*"/>
<ColumnDefinition Width="1*"/>
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="74*"/>
<RowDefinition Height="74*"/>
<RowDefinition Height="421*"/>
</Grid.RowDefinitions>
<Border HorizontalAlignment="Stretch" VerticalAlignment="Stretch"
BorderBrush="Black" BorderThickness="1" Grid.Row="2">
<StackPanel VerticalAlignment="Stretch" HorizontalAlignment="Stretch">
<Label Content="Log in"/>
</StackPanel>
</Border>
</Grid>
Even so, like others mentioned, some other panel almost definetely will be better in this case.