Unset images sizes for all OptinMonster optin themes, install as plugin.

optinmonster-image-sizes.php

<?php
/**
 * Plugin Name: OptinMonster Unset Images Sizes
 * Plugin URI: http://optinmonster.com/
 * Description: Disable the custom image sizes used in the OptinMonster themes. Optins will use full image size instead.
 * Version: 1.0.0
 * Author: Erica Franz
 * Author URI: https://fatpony.me/
 * License: GPL2
 */
 
 defined( 'ABSPATH' ) or die( 'No script kiddies please!' );
 
 add_filter( 'intermediate_image_sizes_advanced', 'ekf_om_filter_image_sizes' );
 function ekf_om_filter_image_sizes( $sizes ) {
 
 	// Unset Lightbox theme image sizes
	unset( $sizes['optin-monster-lightbox-theme-balance'] );
	unset( $sizes['optin-monster-lightbox-theme-bullseye'] );
	unset( $sizes['optin-monster-lightbox-theme-case-study'] );
	unset( $sizes['optin-monster-lightbox-theme-clean-slate'] );
	unset( $sizes['optin-monster-lightbox-theme-pendleton'] );
	unset( $sizes['optin-monster-lightbox-theme-transparent'] );
	
	// Unset After Post theme image sizes
	unset( $sizes['optin-monster-post-theme-banner'] );
	unset( $sizes['optin-monster-post-theme-fabric'] );
	unset( $sizes['optin-monster-post-theme-postal'] );
	
	// Unset Sidebar theme image sizes
	unset( $sizes['optin-monster-sidebar-theme-banner'] );
	unset( $sizes['optin-monster-sidebar-theme-fabric'] );
	unset( $sizes['optin-monster-sidebar-theme-postal'] );
	 
	return $sizes;
	
 }