Follow general disk space requirements for an upgrade. See Chapter 4, System Requirements, Guidelines, and Upgrade (Planning), in Solaris 10 5/08 Installation Guide: Planning for Installation and Upgrade.
The disk on the new boot environment must be able to serve as a boot device. Some systems restrict which disks can serve as a boot device. Refer to your system's documentation to determine if any boot restrictions apply.
The disk might need to be prepared before you create the new boot environment. Check that the disk is formatted properly:
Identify slices large enough to hold the file systems to be copied.
Identify file systems that contain directories that you want to share between boot environments rather than copy. If you want a directory to be shared, you need to create a new boot environment with the directory put on its own slice. The directory is then a file system and can be shared with future boot environments. For more information about creating separate file systems for sharing, see Guidelines for Selecting Slices for Shareable File Systems.