Just search on, Google XML Sitemaps, on the WP backend and it will call it up. There's a few plugins that will generate HTML sitemaps automatically for your users as well.
As stated above, Google Webmaster tools auto detects the gzipped version. Since XML is well-defined, it zips fairly effectively.
If you search through the Google info, they do state a max number of URL's crawled, but am not sure if that is per spider visit or altogether.
Probably worth asking Shenron or Will what methodology they use on their really large sites :)