You can change/increase upload file size setting of course material to be uploaded, by following the steps below:
Step 1- Click on the gear icon
Step 2- Click on Edit settings
Step 3- Scroll down until Files and uploads section
Step 4- Click on the Maximum upload size drop-down to specify the maximum file size to be uploaded in this course
Step 5- Click save and display 
The maximum submission file size for Turnitin Assignment can’t increase more than 100MB as it is Turnitin limitation