Wordpress: add image size

add_image_size.php

<?php
//set custom sizes
if ( function_exists( 'add_image_size' ) ) {
	add_image_size( 'category-thumb', 300, 9999 ); //300 pixels wide (and unlimited height)
	add_image_size( 'homepage-thumb', 220, 180, true ); //(cropped)
} 

//rename custom sizes for the Dashboard, with translation
add_filter('image_size_names_choose', 'PREFIXIT_image_sizes');
function PREFIXIT_image_sizes($sizes) {
        $addsizes = array(
            "new-size" => __( "New Size")
        );
        $newsizes = array_merge($sizes, $addsizes);
        return $newsizes;
}

//unset default wordpress sizes
function PREFIXIT_filter_image_sizes( $sizes) {
		
	unset( $sizes['thumbnail']);
	unset( $sizes['medium']);
	unset( $sizes['large']);
	
	return $sizes;
}
add_filter('intermediate_image_sizes_advanced', 'PREFIXIT_filter_image_sizes');
?>