The other day, Google's John Mueller was asked by an SEO named Torben what the cut off was for the fetch as Google feature. John Mueller responded on Twitter that he knows there is a cut off but was not sure what the number was. So Torben tested it and came up with 10,000 pixels.
Here is his screen shot of the test:
Screenshot of the Cut-Off point at 10,000 Pixel in my test pic.twitter.com/qcq8RtD9wO
— Torben (@TorbenSEO) August 12, 2017
In his German blog post he explained that while the fetch as Google rendering cut off is 10,000 pixels, all of the HTML on that page was fully indexed. So this is just a fetch as Google rendering limitation, not a limitation on what Google can index.
Forum discussion at Twitter.