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First, consider your needs. There's a decent chance that everything you want will fit on the SSD anyway, at least if you aren't an enthusiastic multimedia collector. Nowadays a 1TB SSD can be had cheaply and is likely a much better value for money than a 500GB one; and drives *smaller* than that are somewhat hard to find as the technology has marched on. (If you're using an NVMe SSD on a laptop, the equation may change.)

On the flip side of that, Linux generally installs offline from an image that fits on a 4GB microSD card (or a single-layer DVD) and absolutely (from personal experience) **can be kept running smoothly while fitting everything except `/home`** (and possibly some archived log files) **in a 16GB partition** (although not everyone will want a separate partition, and your distro may well recommend allocating more space than that - this is especially assuming that you don't plan to install a lot of applications).

If you really insist, [only `/etc` really needs to be](https://serverfault.com/questions/578623) on the main partition to have a bootable system, assuming a [reasonably modern](https://systemd.io/THE_CASE_FOR_THE_USR_MERGE/) system. (You'd probably also want an SSD partition for either `/boot` or `/boot/efi`). However, while you might have a system that *boots* quickly by having just this much on an SSD, you'll be much better served if larger components of the OS (such as, say, the desktop environment) can be loaded quickly. **There's really no practical benefit to obsessing over this kind of minimalism**. Even if you want to set up multiple distros side by side, they won't be able to share a whole lot with each other (setting up a common `/usr/local` might be worthwhile, but that's more about neatness than saving disk space.)

So **rather than designating your HDD as representing a specific part of the Linux filesystem, think of it as a place to keep large, seldom-used files** irrespective of that filesystem. That might include video files (i.e. *part of* what you *might* otherwise hold in `/home` subdirectories) and backups (it's good to have your backup on separate physical media anyway). I like to keep [Timeshift](https://github.com/linuxmint/timeshift) backups of the OS on my SSD (in a non-user folder in `/home`), and periodically rsync them (and other specific files I might want to keep backed up) to an external HDD. (Other distros will have similar tools, of course.)