Each slider image set to background image.If you set style(background-size: cover), FlexSlider is now full screen.

flexslider-full-size-image.js

/**
 * Change image tag to this parent background image.
 */
( function( $ ){
	
	$.when(
	
		$.each( $( '.slides li' ),
			function( index, elem ){
	
				var img = $( elem ).find( 'img' );
				var imgSrc = $( img ).attr( 'src' );
				console.log(encodeURIComponent(imgSrc));
	
				$( elem ).css({
					'backgroundImage': 'url(' + imgSrc + ')',
					'backgroundRepeat': 'no-repeat',
					'backgroundSize': 'cover',
					'backgroundPosition': 'center center'
				});
	
	})).then( function() {
	
		$( '.slides li img' ).remove();
	
	});
	
})( jQuery );