When creating an appstack if I notice that the default 20GB template doesn't cut it I simply enlarge the disk there and then. I once needed to extend the appstack 2 times, just enlarge it using the packaging machine and extend in Windows et voila.
If you do have an error with the default 20Gb template without extending my guess is that you have a different issue.
When looking at your use case you need to have the applications to be published using RDSH? Because you are packaging on a Server 2012? You do know that you need to remove View Agent before packaging? Do you happen to have any kind of logging or screenshots about what is happening? When is the error showing up? Are you able to start the packaging process?