Hi Elizabeth,
What type of file are you uploading? I just uploaded a 144 page PDF succesfully as a document for a Document Based Assessment. I saved it onto my desktop and uploaded from there. Try using a different method, google drive if you haven't tried that, or Dropbox. Let us know how it goes and if you need more help.
Here are the directions to creating a document based assessment, to check to make sure you're following the correct steps.
