By default, student uploads for any kind of submission are limited to 50MB. If you want to them to upload something that may be larger, such as a PowerPoint slide deck or video file, then you can change the settings for the course. The settings to do this are not in the Assignment tool, but rather in the Course administration settings. These settings can be accessed by clicking Edit settings either in the Settings button in the upper right corner of the main page, or in the Administration block on the right side of the page, below Quickmail.

On the Edit course settings page, scroll down to the Files and uploads settings. The only entry is Maximum upload size. Change this to a size that is more appropriate for the expected uploads but note that the total site upload limit is 500MB.

To complete the change, click Save and display at the bottom of the page