by ben reynolds.
You are mistaken. The maximum file size in Assignment (& elsewhere) can be adjusted to "Activity Upload Limit," the top item in the "Maximum submission size" dropdown in "Submission types" in editing Assignment.
That activity limit is set in Course Administration > Files and Uploads > Maximum upload size.
The course limit is set at the Site Level. Administration > Security > Site Policies > Maximum uploaded file size
The documentation is here https://docs.moodle.org/29/en/File_upload_size
Above Moodle, there is a php.ini file size, and, if you are on a Windows server, the file size needs to be set in Windows server as well.