The Handbook gives very minimal information about how large the root partition should be if one is not using the "rest of the disk" for it, as shown in the default partitioning scheme. It mentions that /usr/portage will be "around 650 MiB" (mine is currently 711MiB, not counting distfiles), but gives no indication whatsoever of how large the entire rootfs will end up being. (When I first tried installing Gentoo many years ago, I made the mistake of using a root partition too small to even unpack the stage3 tarball!)
I just checked, and the latest amd64 multilib stage3 tarball (a 183 MiB file, as downloaded) when unpacked rolls out to a generous 1046 MiB. Since I'm not actually installing a new Gentoo system at the moment, I have not checked how big everything is after completing the entire installation process. (Obviously this is highly dependent on the architecture and system configuration choices the user has made, but just having that info for one particular system would be helpful, I think, to give readers some idea of what to expect.)
If I were to complete the install based on that stage3, I estimate the final filesystem (upon completion of the Handbook) would run to at least 2.7 GiB (stuff mentioned above plus a gentoo-sources kernel tree).
Of course, after adding a desktop (which most users want) and various other software, a truly "complete" system can be several GiB larger than that. Personally, my current root filesystem — with OpenRC, X, MATE, Firefox, Thunderbird, 2 versions of GCC, 2 gentoo-sources kernels, and misc other sotware (of course), plus the distfiles from my last emerge --emptytree world (followed by an eclean-dist --deep) — stands at 8.8 GiB used (not counting /home).
Users should therefore be warned in the "Designing a partition scheme" section that just unpacking the stage3 tarball can take more than 1 GiB, the portage tree — er… Gentoo repository — (at /usr/portage) is over 700 MiB (and likely to grow), each kernel tree (under /usr/src/linux/) can take up to 1 GiB (or possibly more), the X graphical system takes… some amount of space, and a full-blown desktop environment can run to… however big.
We should also warn users at this point in the Handbook that certain large software packages (e.g., Thunderbird) can take 4 GiB or even 8 GiB of free space to build (the latter if pgo is used), which by default happens at /var/tmp/portage — and that /usr/portage/distfiles can get very large (2.5 GiB or more), even if you purge all old distfiles not associated with an installed package.
Some of this may be touched on in other parts of the Handbook (I can't remember), but it needs to be said at the point where users are partitioning their drives.