So long as the new 20Gb drive is compatible with the Ultra5, I would reinstall from the CD and do a complete reinstallation on the new disk. Formatting is then done by the installation process.
OK>boot cdrom
should do it
Newfs is a shell command not an OBP (OK Boot Prompt)command, so unless you boot to a shell prompt from CD (or as a diskless client or network boot) you won't be able to use it. But unless you have a specific reason for not using the installation process formatting, such as a specific set of parameters, like block size, that you want your boot disk to use, it's much easier to go with the defaults.
If you have files that you wish to transfer over from the old disk (with no tape drive/CDRW/etc present) I would attach the old disk back into the machine after installation and mount it as a different file system under the new file structure.
If there is not much info on the old disk it's probably easier just to note down the relevant info e.g. IP, hostname, domainname, etc and use it when prompted on the new install.
Hope this helps. Ian
"IF" is not a word it's a way of life