1. Check for “upload_max_filesize” in Site Administration > Server > PHP Info. Not the value you wanted? Change it in your server php.ini (Site Administration > Server > PHP Info > Loaded Configuration File).

You may want to alter the following settings:

memory_limit
upload_max_size
post_max_size
upload_max_filesize
max_execution_time
max_input_time

2. Site Administration > Plugins > Activity modules > Assignment > Submission Plugins > File submissions.

3. Site Administration > Courses > Course default settings > Files and Uploads section.

4. Your Course area: go to Administration > Edit Settings > Files and uploads.

 

Source:
[https://www.colourmylearning.com/2014/06/how-to-set-assignment-upload-limit-in-moodle/]