You might need to use grid layout or other to make the controls fluid in Design. To add controls to the grid layout panel just put the declaration between the opening and closing tags of the Grid. Keep in mind that the row- and columndefinitions must precced any definition of child controls.
The grid layout panel provides the two attached properties Grid.Column and Grid.Row to define the location of the control. As an Example you might try with the following given code. If you count in terms of percentage you just remember that 100% means 1. So if you have 3 controls and you want to put it in three column then you might design its width as .33* for all
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="Auto" />
<RowDefinition Height="Auto" />
<RowDefinition Height="*" />
<RowDefinition Height="28" />
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto" />
<ColumnDefinition Width="200" />
</Grid.ColumnDefinitions>
<Label Grid.Row="0" Grid.Column="0" Content="Name:"/>
<Label Grid.Row="1" Grid.Column="0" Content="E-Mail:"/>
<Label Grid.Row="2" Grid.Column="0" Content="Comment:"/>
<TextBox Grid.Column="1" Grid.Row="0" Margin="3" />
<TextBox Grid.Column="1" Grid.Row="1" Margin="3" />
<TextBox Grid.Column="1" Grid.Row="2" Margin="3" />
<Button Grid.Column="1" Grid.Row="3" HorizontalAlignment="Right"
MinWidth="80" Margin="3" Content="Send" />
</Grid>