Bing has announced a change to the sitemap file size limit, increasing it from 10MB to 50MB.  Google has also confirmed they are honoring the new sitemap size increase too.

It is important to note that the change doesn’t increase the actual number of URLs in the sitemap file.  That is still limited to 50,000 URLs.  But it does increase the file size to take into account some of the other aspects of sitemaps that are bringing the sizes of these files up, such as hreflang, which can bulk up sitemap file size quickly.

They also note that if you are using gzip to decrease the size, when it is uncompressed it still must be under the new 50MB limit.

The sitemaps.org site has also been updated:

Q: How big can my Sitemap be?

Sitemaps should be no larger than 50MB (52,428,800 bytes) and can contain a maximum of 50,000 URLs. These limits help to ensure that your web server does not get bogged down serving very large files. This means that if your site contains more than 50,000 URLs or your Sitemap is bigger than 50MB, you must create multiple Sitemap files and use a Sitemap index file. You should use a Sitemap index file even if you have a small site but plan on growing beyond 50,000 URLs or a file size of 50MB. A Sitemap index file can include up to 50,000 Sitemaps and must not exceed 50MB (52,428,800 bytes). You can also use gzip to compress your Sitemaps.

The change is now active.