murbert There should be 2 recommendations
1 - Minimum required for successful installation and operation (thinking about those who will browse the web, play some solitaire and likely never do much installing, like my mom)
2 - Average user recommendation
I think that this make sense.
MINIMUM
I took the time last night and this morning to install OTB installations of Solus Budgie, Solus Gnome, Solus MATE and Solus Plasma on my 11-3180, and, although there were variations in the EXT4 space used between the various DE's, the variations were not significant, varying from just over 10 GB to just under 12 GB.
Based on that, it seems to me that a reasonable benchmark for a minimal installation, allowing 20% free space on the disk in line with best practices, comes out something like this:
Minimal Solus Installation
500 MB EFI partition
4 GB Swap partition
12 GB used on EXT4 partition for installed OTB Solus
5 GB for cache and data
3 GB for best practices free space
TOTAL = 24.5 GB
Rounding up, that suggests that 25 GB is a reasonable requirement for a bare-bones, minimum installation.
AVERAGE DESKTOP USER
Many users would quickly bump up against 25 GB.
An realistic "average desktop user" estimate is a lot trickier, because the estimate depends on what the user installs/uninstalls over time and how much data the user has on the computer. However, from what I've read in this thread and others than I looked at last night, my guess is that an "average user recommendation" between 50-60 GB would be about right.
Just looking at my own experience (similar to @johano's), I'm using 41 GB on the EXT 4 partition with a reasonably lean installation. With 1 GB for cache and whatnot (I'm maniacal about not keeping caches, so my installation is light on cached data) and 8 GB to get best practices 20% free space, that means that I need roughly a 50 GB drive for a working installation.
Other "average users" might need more or less, and "power users" can be expected to use well over 100 GB. I'm starting to see why Ubuntu Budgie recommends a 60 GB minimum.
MODERN HOME COMPUTING
Reading other threads touching on this topic this morning, I was struck by a comment that Josh made: "We target modern home computing devices and we really do mean that." Josh went on to explain that "Low end systems are not the target of Solus ..." and explain why in some detail.
Solus is not designed for or targeted at low-end or old equipment, and that is not going to change, as at least some users are likely to find out this weekend when the linux-lts kernel cuts over from 4.14 to 5.15. Solus can only do so much to accommodate low-end or old hardware.
Along those lines, most modern computers have plenty of disk space to support Solus. As we have discussed in this thread and others, with the exception of Chromebooks, almost all computers from major manufacturers come with a minimum of 128 GB at present (and those are getting rare - 500 GB seems to be the emerging norm), and almost all computers sold in the last 6-8 years have at least 64 GB, except for older Chromebooks and a few, very low-end, Windows laptops.
Be that as it may, Solus is not Puppy, Lubuntu or Linux Lite, and need not pretend to be. Although Solus will run on 32 GB hardware, Solus is not a good fit for low-end or old hardware in many respects. That's just a fact.