<?php
function pw_add_image_sizes() {
    add_image_size( 'pw-thumb', 300, 100, true );
    add_image_size( 'pw-large', 600, 300, true );
}
add_action( 'init', 'pw_add_image_sizes' );
 
function pw_show_image_sizes($sizes) {
    $sizes['pw-thumb'] = __( 'Custom Thumb', 'pippin' );
    $sizes['pw-large'] = __( 'Custom Large', 'pippin' );
 
    return $sizes;
}
add_filter('image_size_names_choose', 'pw_show_image_sizes');

for adding custom image sizes you can select from media uploader

Credit: pippinsplugins.com/add-custom-image-sizes-to-media-uploader/

Adding Custom Image Sizes

PHP