What Jeff means, is not within the DA Control Panel, and not with any other DA specific tools.
However, you certainly could setup a backup script using your command-line (access via SSH). Something like RSync could be used and you could check folder sizes using BASH. The script could be then added to your crontab and run periodically (daily for example).
When you write a backup script, you need to backup "everything" about the user, not just the /home/<user> folder. For example, there are databases, log files and custom settings too.
It may be worth asking for DA to add admin_backup to the API - that way custom backup scripts can do things like testing <user> folder size and run the individual backup. Then all you have to do is write a simple BASH script, and the DA backup does all the hard work.