According to the website, backups for Cloud Servers are only available for instances with a memory size of 2GB or less. However, this restriction only applies to (some of) the control panel, and not the API itself. It is still quite possible to take snapshots of your larger servers, though there are a few caveats along the way.
To take snapshots of your >2GB Cloud Servers through the control panel, you first need to go to Hosting -> Cloud Servers and click on the “My Server Images” tab. Note that you do not want to go to an individual server’s overview page to take the image; it will not work for >2GB servers.