How can I tell CWM to perform a full backup instead of an incremental backup?
Technically, CWM 6 isn't doing incremental or full backups. What it is doing is storing "blobs" that are shared between backups if the data hasn't changed between the two. When you run a new backup, it reuses what it can. If you delete a backup set, you don't actually delete the data, and the next time you run a backup, CWM deletes any blobs no longer in use.