No, it does not follow any rule of thumb. If it did, my page file would be a whopping 64GB. But it is currently set to 4186MB
Ummm, not true. First, the size needed is not determined by the way the OS "manages" memory. The size needed is determined simply by the size of the files that make up the OS.
It was a joke from my part, that's why smilie to not make confusion.
Recap, after my last reply, my pagefile grow to 18003 GB, RAM is 71% full. I don't have anything else but browsers running with more or less tabs open and running in background, I should check extensions or add-ons hibernating or suspending tabs in background like Vivaldi browser is doing out of the box I think. I would expect Windows would flush unused garbage from pagefile, but no, it keeps growing.
Beside crash dumps or something and hibernation there is no reason to fill swap or pagefile if there is enough RAM, because RAM is the fastest memory available and in old times when there were only mechanical disks it would be terrible to use HDD as RAM alternative, it would be like a crutch RAM. If you have large enough RAM I don't see a reason memory management would not manipulate everything in RAM instead on more slower mechanical drive or SSD.
AmigaOS and Atari TOS could use RAM disk for a reason and I remember by the way, at least on Atari ST, you could use hardware reset button and program (Macintosh emulator if memory serves me well) would stay resident in memory and you need to completely turn off computer (hardware button) to clear off program from memory.
Speaking of which Gigabyte released i-RAM long time ago, it was a PCI card with DDR RAM slots serving as RAM disk, way ahead of its time considering low density or low capacity of DDR.
I thought that 16 GB of (DDR 4) RAM would be enough for browsing and using of computer casually when I saw how 8GB of (DDR 3) RAM on my other computer configuration was not enough. I tend to have many tabs open in browsers and propably many casual users never experienced my case scenario.
Desktop PC is not a server, one size does not fits all. For servers swap or pagefile have a meaning, on desktop (or workstations) I dare to say, not as much. I don't know how many desktop users and professionals would check out logs and crash dumps for debugging or something.
Technically, you only need
20GB for 64-bit Windows 10 (though a minimum of 32GB is recommended). However, I would never recommend anything less than a 128GB drive for the OS as that gives Windows enough room for drivers, temporary files and the PF. But I would recommend a secondary drive for all installed applications if the boot drive is less than 250GB.
You said it yourself. You recommend at least 128GB drive for the Windows and I say 40-60GB SSD or even lower is enough for any GNU/Linux or FreeBSD distibution. I was forced to buy 256GB SSD because everything less was pain in the ass for using Windows 10 and now, even 256 GB is to small. If you need Visual Studio and you have 256 GB SSD you are screwed, 500 GB (NVMe) SSD here I come.
That said you can easily move your temp files location, Documents folder, and the PF to a different drive in Windows too. So your point there is invalid.
I moved default \Documents, \Downloads etc. folders to another D: drive. Having permission issues for some of those folder or files inside of them after fresh install is another problem despite using same account. Maybe I did something wrong, can't remember and it's another story.
And for the record, a quick bit of homework with Google shows the minimum system requirements for
Ubuntu Linux calls for 25GB of disk space - 5 more than W10!
No its not! A little bit of homework and setting aside of biases is needed here.
Firstly, it was not my intention going into GNU/LInux/Unix/FreeBSD vs Windows flame war. It was just example how things works or it could work as alternative point of view.
GNU/Linux got its own fair share of stupidity and complications, such as SysV vs systemd and forkifications for the sake of it and ego trips.
Considering Ubuntu recommendation, it is propably safe bet for noobs, because I know my / (root) partition never get bigger from 20GB or less no matter what distribution I use, not to mention FreeBSD.
And you can be sure those 25GB will never be a problem for GNU/Linux user, because Windows user will need more than 20GB of space for Windows normally to run if everything is on default after installation.
Consider this. There are over 1.6 billion (with a "b") Windows computers out there. Virtually each and every one became a unique system within the first few minutes of being booted up the very first time! Users setup their accounts, personalization, networking, security apps, personal apps, and peripherals. And Windows supports them all.
Argumentum ad populum.
If we count smartphones, supercomputers and servers, Linux or Unix like OSs serve even more users but this is irrelevant for discussion.
But I get your point, it is complex to make one size fits all solution.
By the way, Microsoft had opportunity with their WP (I own Lumia 640) to be good alternative for iOS and Android, but they blew this opportunity up by own idiocy and this is problem for big companies, they become slow to change some things and lose their focus. Just like IBM management did not know what to do with PC and Atari or later Commodore blew up Amiga project.
With Windows, you can buy an ASUS motherboard, AMD processor, MSI graphics card, Western Digital hard drive, 8GB of Kingston RAM, put them in a Corsair case, power them with a Seasonic power supply, connect it to a 27" Acer monitor and Epson laser printer, connect to your network via Ethernet, install AVAST security, and install Microsoft Office on it to create your resume/CV , and it will work.
Or you can buy a Gigabyte motherboard, Intel processor, XFX graphics card, Samsung SSD, 16GB Crucial RAM, and put them in a Fractal Design case, power them with an EVGA power supply, connect them to two 24" LG monitors and HP ink jet AiO, connect to your network via wifi, use Windows Defender and install LibreOffice on it to create your resume/CV, and it will still work.
You mentioned apples and oranges. Some of this stuff is about standards and some of them are about compatibility. TCP/IP, CUPS, .DOC you know.
I don't use any other antivirus program beside default Windows Defender since it became an option. I used Avast and Avira (on XP and Win 98) long time ago, AVG never liked it and others more known (NOD32, Kaspersky, BitDefender, you name it) were shareware or something with less options available.
By the way, Apple before Steve Jobs came in charge again, tried this approach, by making Macintosh clones. You've had Radius (specialized in Macintosh peripherals and accessory equipment) Macintosh clones among others.
Same apply for GNU/Linux and FreeBSD and some other more "exotic" OSs, like HaikuOS. You need to comply with certain standards and you don't have a problem with compatibility.
And you could buy AMD (there were also Cyrix and NexGen in old times) instead of Intel to run x86 instructions. Fortunately, thanks to advancing in process, technology and software development there is rise in OpenRISC architecture and similar so people/users will have another alternative.
There is a reason PC is/was called IBM/PC compatible. All those manufacturers follow certain standards required to run IBM/PC compatible computers.
Cases are irrelevant here (you can have computer (hardware) without them but it is not practical and safe), they need to follow certain formats, ATX, EATX, ITX etc. They are just boxes where you put your hardware, no pun intended.
If you are a Toyota mechanic working in a Toyota dealer's service center, and you see nothing but broken down Toyota's all day, if you don't keep an open mind and set aside any preconceived notions, you could easily start to believe Toyota makes lousy cars.
Usually automotive and computer analogy are awkward and lousy at best but are useful or simple to describe something to technologically inept people.
Cars don't change like hardware and software IT industry, especiallly not as in Moore's law otherwise we would have flying cars. You have the same working principle in engines be it petrol, diesel or electricity. There are small variations, but you don't have so much space in advancing as in IT industry.
For sure if you are Toyota mechanic repairing only Toyota cars every day you could get wrong impression, but you could share experience with other mechanics repairing other cars so you could make some kind of comparision.
It is same with laptops. Some laptops could have more returns than others, but you need to see if this is result of more usage (more buying and consequently more returns) or really they are more prone to failure.
My bad! I clearly should have picked up on that. Obviously I am not fully caffeinated yet today so I'm not good at reading facial expressions and body language, or hearing tone of voice via the forums! 
Yes, I am always right - except when I'm wrong. And that happens. But I tend to practice what I preach in this regard and do my homework before posting so it doesn't happen often.
Why would you expect (or like) the Page File on Windows to be sized the same as the swap file on GNU/Linux? They are totally different operating systems and surely you were running totally different programs. Frankly, I would be surprised if they were the same size.