Hello friends,

How to create a sitemap for a very large site..?

Yes you can, but it is not at all easy.

Create an XML sitemap that can be submitted to Google, Bing and other search engines to help them crawl your website better.
Create a Text sitemap to have a plain list of all your pages.
Generate an HTML site map to allow website visitors to easily navigate on your site.

You can use a paid tool to create the sitemap for a very large sire.

You can use paid tool for creating sitemap for large websites

In my suggestion u have to use a paid tool for creating large website

How large is your website? Try the trial of A1 Sitemap Generator and see if it solves your problem

xml sitemap generator has free but has limited urls to be stored and for a very large website you will need premium account.