Remove image sizes in WordPress. Source: http://www.wpmayor.com/remove-image-sizes-in-wordpress/

custom-image-sizes.php

<?php

function wpmayor_custom_image_sizes($sizes) {
    $myimgsizes = array("image-in-post" => __("Image in Post"), "full" => __("Original size"));
    return $myimgsizes;
}
add_filter('image_size_names_choose', 'wpmayor_custom_image_sizes');

remove-image-sizes.php

<?php

//* Remove image sizes in WordPress
function customprefix_remove_image_sizes($sizes) {
    unset($sizes['medium']);
    unset($sizes['large']);
    unset($sizes['plugin-name-image-size']);
    return $sizes;
}
add_filter('intermediate_image_sizes_advanced', 'customprefix_remove_image_sizes');