Modify Maximum upload file size text

This text is coming from wp-admin/includes/media.php#L1946

There is not filter is available to modify the text. But still if you wish you can use gettext filter to modify the text.

add_action('post-html-upload-ui', function () {
    add_filter('gettext', 'media_upload_limit_custom_text');
});
/**
 * Customize the max media size text
 * @param string $text
 * @return string $text
 */
function media_upload_limit_custom_text($text) {
    if ($text == 'Maximum upload file size: %s.') {
        return __('Image files must be smaller than 5000 KB', 'your-text-domain');
    }
    return $text;
}

We are adding gettext filter just before our text is being display!