Prevent large image uploads

You have a few different solutions available here:

Automatically scaling down

If you just do not want to store huge amounts of image data on your webspace, I’d recommend the Plugin Imsanity. This automatically scales down the uploaded images, even if they are too big.

Forbid large uploads

In this case the user has more work to do, as they will have to scale down the images on their own. You could filter the wp_handle_upload_prefilter:

add_filter('wp_handle_upload_prefilter', 'f711_image_size_prevent');
function f711_image_size_prevent($file) {
    $size = $file['size'];
    $size = $size / 1024; // Calculate down to KB
    $type = $file['type'];
    $is_image = strpos($type, 'image');
    $limit = 5000; // Your Filesize in KB

    if ( ( $size > $limit ) && ($is_image !== false) ) {
        $file['error'] = 'Image files must be smaller than '.$limit.'KB';
    }

    return $file;

}

Change the PHP values

This one is fairly straight forward, you just have to set max_upload_size in your .htaccess or php.ini.