deny-giant-image-uploads.php

<?php
/**
 * Plugin Name: Deny Giant Image Uploads
 * Description: Prevents Uploads of images greater than 3.2MP
 * Author: TJNowell
 * Author URI: http://tomjn.com
 * Plugin URI: http://tomjn.com/164/clients-who-upload-huge-camera-photos-decompression-bombs/
 * Version: 1.1
 */

function tomjn_deny_giant_images($file){
    $type = explode('/',$file['type']);

    if($type[0] == 'image'){
        list( $width, $height, $imagetype, $hwstring, $mime, $rgb_r_cmyk, $bit ) = getimagesize( $file['tmp_name'] );
        if($width * $height > 3200728){ // I added 100,000 as sometimes there are more rows/columns than visible pixels depending on the format
            $file['error'] = 'This image is too large, resize it prior to uploading, ideally below 3.2MP or 2048x1536';
        }
    }
    return $file;
}
add_filter('wp_handle_upload_prefilter','tomjn_deny_giant_images');

Thank you for the inspiration: https://github.com/szepeviktor/wordpress-plugin-construction/blob/master/mu-image-upload-control/image-upload-control.php