The Google disavow file has a 100,000 URL and 2MB file size limit. I am not sure if this is documented, but it is known in the industry.
One webmaster posted in the Google Webmaster Help forums that his disavow file wasn’t working. John Mueller came in and explained that there is a limit to the size of the file. He said “The disavow file has a limit of 2MB. You should see a warning similar to this when uploading a file that’s too large.”
The warning should read:
The uploaded file is too long. It must have fewer than 100000 lines and must be less than 2MB in size.
But this webmaster is only seeing:
Forum discussion at Google Webmaster Help.