I created sitemaps for a large site. The generator stopped a few times but I was able to restart. The finished sitemaps show 78548 pages indexed. There are 3 files, sitemap.xml.gz, sitemap1.xml.gz and sitemap2.xml.gz.
When I submitted to google, an error message came back saying the file size was too big, remove some urls. I clicked on their help icon and it said that a maximum of 50,000 urls is allowed (not file size).
The sitemap1.xml.gz file contained 50,000 urls. I cut out 10,000 and pasted them into sitemap2.xml.gz and resubmitted. I just received google's reply, accepted OK.
I believe the client program gsitecrawler creates max 40,000 url sitemaps. Previously I had used gsitecrawler, and with a large site there were too many problems, all that data has to be downloaded over the net. xml-sitemap generator is much faster. What took over a week is now completed in a few hours.