When you install WordPress Multi-site, it allows you to impose a media file limit for users. Whatch this video, or see the notes below.
This is a great feature when running multisite for a big community and where you want to keep their media file content under control. But, the default size is quite small and might not be suitable for your use.
The global setting for all WordPress Installations is stored in your php.ini file. We suggest you speak to your hosting support for the limits they allow and how to change the value.
You can however change the value from the network settings. Scroll down the the bottom of this section and you will see values to limit individual file uploads, and the total size of a media folder for each WordPress installation. Make sure you save changes.