Your placement of the Spacer Box and the size of the Spacer Box is all that needed to be changed to get your code to work

Names Default To Here( 1 );
Open( "$SAMPLE_DATA/Big Class.jmp" );
New Window( "Spacing",
	Lineup Box( N Col( 2 ), Spacing( 0 ),
		Variability Chart( Y( :height ), X( :age, :sex ), Std Dev Chart( 0 ) ),
		V List Box(
			Distribution(
				Continuous Distribution(
					Column( :height ),
					Quantiles( 0 ),
					Summary Statistics( 0 ),
					Horizontal Layout( 1 ),
					Vertical( 0 ),
					Outlier Box Plot( 0 )
				)
			),
			Spacer Box( size( 1, 150 ) )
		)
	)
);
					
				
			
			
				
	Jim