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:

But this webmaster is only seeing:

Forum discussion at Google Webmaster Help.