Follow general disk space requirements for an upgrade. See Chapter 5, Guidelines for Allocating Disk Space and Swap 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.