The control domain and the guest domain are separate entities.
Each domain has a name and a UUID. Domains can be renamed but typically retain the same UUID.
A domain ID is an integer that is specific to a running instance. The ID changes whenever a guest domain is booted. A domain must be running to have a domain ID.
Control Domain 0
The control domain is a version of Solaris modified to run under the xVM hypervisor. When the control domain is running, the control tools are enabled. In most other respects, the control domain 0 instance runs and behaves like an unmodified instance of the Solaris Operating System.
Other than by login, you cannot access a guest domain from the control domain. A control domain should be reserved for system management work associated with running a hypervisor. This means, for example, that users should not have logins on the control domain. The control domain provides shared access to a physical network interface to the guest domains, which have no direct access to physical devices.
If a control domain crashes with a standard Solaris panic, the dump will include just the control domain. Also see About Crash Dumps.
Control domain 0 space requirements are provided in the Solaris Express Release and Installation Collection on docs.sun.com.
You will need to allocate sufficient disk space at the time the Solaris control domain system is installed. A full Solaris install (SUNWcxall) consumes approximately 4 Gbytes of disk space.
Guest Domain Space Requirements
Domain U is a guest, or unprivileged, domain.
Size your domain as you would configure a machine to do the same workload.
The virtual disk requirement is dependent on the guest operating system and software groups that you install.