I have available space, but I can't upload a .5MB file to the Document Cloud.

Free up some space.  Permanently deleting files is a 2-step process.

• https://helpx.adobe.com/creative-cloud/kb/file-storage-quota.html
• https://helpx.adobe.com/creative-cloud/help/delete-files-permanently.html

In What Web App are you trying to use for this?

Where it the file located on YOUR Computers hard drive? How much space is available on the drive?

Some Links that may help... all the links I have, since I don't know the cause of your specific problem
-FAQ https://community.adobe.com/t5/Get-Started/Helpful-Links-FAQ-Where-can-I-find-links-about-managing-and/td-p/7570118
-http://helpx.adobe.com/creative-cloud/help/sync-settings.html
-http://helpx.adobe.com/creative-cloud/kb/arent-my-files-syncing.html
-sync and email link https://community.adobe.com/t5/Get-Started/Email-link-of-file-in-personal-CC-folder/td-p/5944719
-an overview of assets https://assets.adobe.com/files
-File sharing https://community.adobe.com/t5/Get-Started/Does-Creative-Cloud-support-sharing-videos/td-p/7113392