Direct backup from Titanium Backup to DropBox
I don't think that's possible. TiBu syncs a local copy to DropBox.

What you could do is: 


Use / Buy an external SD card (64GB or less)
Go to System settings, open the Application manager, and Move to SD card TiBu


This way, it doesn't take up internal phone memory. (i.e. TiBu dumps its backup to the external SD card)