In a joint announcement between Google and Bing, the companies have stated they are increasing the limit of sitemap file sizes from the standard 10 MB to 50 MB.
Most sitemaps never reach the 10 MB file size limit, although larger sitemaps are still encountered on occasion. This can happen when sitemaps contain very long URLs, or if the sitemap contains attributes with long URLs such as image URLs, alternate language URLs, and so on.
Now larger sitemaps will no longer be a problem, as long as they stay under 50 MB. It’s worth noting that sitemaps can still be compressed using gzip if you need to reduce the file size based on your bandwidth limit. However, once unzipped the sitemap still cannot exceed 50 MB.
It’s also important to note that this change impacts sitemap file size only. It does not affect the limit on the amount of URLs that can be contained in a sitemap, which is still 50,000. Limiting your sitemap to only include relevant pages is recommended not only to keep it under 50,000 URLs, but it’s a recommended SEO best practice as well.
Keep in mind that although your sitemaps can now be up to 50 MB in size, smaller sitemaps can prevent your server from becoming overloaded. If Google is frequently requesting a 50 MB sitemap it could end up putting unnecessary strain on your server.
For more information on best practices, included advice for large sites, see this post on sitemap best practices.