10 mb or 50,000 URls. more than that, use index files and separate sitemaps. http://sitemaps.blogspot.com/2005/08/using-sitemap-index-files.html
In my opinion sitemap links vary according to individual opinion. I suggest one site map contain only 2000-2500 links. If your site contain more link then made a second site map.
I think, you can split the sitemap into different pages if they have too many urls and see that each page contains not more than 1000 links. Then go to the next page in the sitemap, promptly interlinking them.
You can provide multiple Sitemap files, but each Sitemap file that you provide must have no more than 50,000 URLs and must be no larger than 10MB when uncompressed. These limits help to ensure that your web server does not get bogged down serving very large files. f you want to list more than 50,000 URLs, you must create multiple Sitemap files. If you anticipate your Sitemap growing beyond 50,000 URLs or 10MB, you should consider creating multiple Sitemap files. A Sitemap index file can list up to 1,000 Sitemaps.