The default maximum size for a single file uploaded by one of your students is 100 MB. This should be sufficient if your students will be submitting images, PDFs, and word processor files. If you will be accepting assignment files within Moodle that are videos, lengthy audio files, or other large files, you will want to raise this limit to 1 GB per file, which is as high as it can be raised.
This change can only be made by the Instructor and is done in the settings for the Moodle course. From your course front page, go to Administration > Course Administration > Edit settings. Then scroll down to the section on "Files and uploads." Use the "Maximum upload size" drop-down menu to choose the file size limit of your choice. Scroll down to the bottom of the screen and click the "Save and Display" button to make your changes take effect.
This setting will pass on the new file size limit to any activities created after it is set. If you plan to accept larger submissions for your class, set your maximum file size to 1 GB before you set up assignments.
If you would like to change the limit in a single assignment, click on that assignment and go to Administration > Course Administration > Assignment Settings. In the Submission Types section of the settings you will be able to raise the Maximum Submission Size.