Limitation for google sitemap.xml file size

According to the Latest Update: (as of 2016):

Any Sitemap file is limited to 10MB (uncompressed) with a maximum of 50,000 URLs.
And a Sitemap index file can include up to 10,000 Sitemaps.
So we can have around 50 millions URLs in separated sitemap files.

in terms of URLs:

49,000 URLs + 1,000 sitemaps
= 49,000 URLs + ( 1,000 * 50,000 ) URLs
= 50,049,000 URLs 

in terms of size:

10MB + (1,000 sitemaps * 10MB)
= 10,010 MB
= 10.01 GB 

So altogether, we can have around 10GB of storage for all sitemap files.

Reference: 

Sitemap formats : Google Webmaster Console
Google supports several sitemap formats such as XML, RSS, mRSS, Atom 1.0 and Text.

All formats limit a single sitemap to 10MB (uncompressed) and 50,000 URLs. If you have a larger file or more URLs, you will have to break your list into multiple sitemaps. You can optionally create a sitemap index file (a file that points to a list of sitemaps) and submit that single index file to Google. You can submit multiple sitemaps and/or sitemap index files to Google.

How big can my Sitemap be? : Sitemaps.org
 Sitemaps should be no larger than 10MB (10,485,760 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 10MB, 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 10MB. A Sitemap index file can include up to 1,000 Sitemaps and must not exceed 10MB (10,485,760 bytes). You can also use gzip to compress your Sitemaps.

Ref: http://stackoverflow.com/questions/2887358/limitation-for-google-sitemap-xml-file-size/34972582#34972582