Follow general disk space requirements for an upgrade. See Chapter 5, Guidelines for Allocating Disk Space (Planning).
To estimate the file system size that is needed to create a boot environment, start the creation of a new boot environment. The size is calculated. You can then abort the process.
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.