I would not use a 128GB for my OS, and even a 256GB would give me pause.
I think you’ve missed my point… What seems like a safe size for your OS now probably won’t be in a couple of years. When I bought that 30GB drive it was more than double what the OS needed. Within a few years the 60GB replacement wasn’t enough.
128GB seems great now. And it is definitely more than enough. But I wouldn’t count on that for long. I’m using a 1TB drive for my OS now. An SSD that size is still not cheap.
I’m a little baffled by your descriptions of OS bloat. I’ve never had anything similar with XP, 7, or 8. I had a 128 GB SSD as the primary drive in my desktop and laptop for almost four years. That was only cramped in the sense that I could only install a handful of games from my Steam library. While the OS did grow due to updates and such, it was more on the order of growing from 8 to 12 gb.
Your OS doesn’t take up that much space. It’s your other apps and/or media that’s doing it – the kind of stuff that could fit on a separate HDD or external drive.
Do not conflate poor choices with poor components. 2x anticipated OS size was a very questionable choice. Blaming the SSD for your issues is flat out wrong.
Where I am asked to do separate partitions for the OS and data I usually leave 100-150GB on the OS side alone with the option to expand the partition if need be.
Wrong. My programs, files, even profiles were on a larger drive. This was all junk from Windows 7 updates. WinSxS alone was a good 30 GB+.
I’m not saying I didn’t make bad choices… I did make bad choices! 
I’m warning people away from my mistakes.
I haven’t experimented with hybrid drives yet. I’ve considered trying one for an OS install, has anyone here used one before? I haven’t seen any used in my professional life or seen one on a privately-owned machine. I know Macs were coming with them as standard.
Not good for an OS. Apparently, an OS keeps needing small bits & bytes everywhere so you need the whole OS to be on solid memory. Hybrid drives typically only have 8GB of solid memory. Plus, the content of that solid memory changes as the drive learns which programs you use more.
Hybrid drives seem great if you commonly use a bunch of programs which it would be prohibitively expensive to put on an SSD.
Looking at Newegg, there’s only one hybrid drive rated 5/5 and that’s only based on 1 review. Above 1TB, they seem to have reliability issues which is death for a storage medium.
I can’t fathom requiring even 128GB for just the OS.
I have a 256GB SSD drive and about half is in use (plus a HD 1TB for data). On it is the OS (MS-Windows 7), programs (and I have a lot of programs, including a big Cygwin install), plus a bunch of data from years gone by I never bothered to move to another partition. You can do a clean install of recent MS OSes in 64GB and have echo chamber room left over. You’re going to need a lot of space for other stuff, but not the OS.
If you want to talk about how much space you need, please don’t just say it’s for the OS. It isn’t.
(Just checked, my Windir is only 14.4 GB. That includes some non-OS stuff but there’s also OS related crap tucked away in other places. Note that recommended disk sizes on boxes include space for the OS plus typical programs.)
I got used to the multi-partition model of managing a computer way, way back. If you are comfortable with this, then having a modest SSD drive for OS and key programs will do just fine. But too many people think C: = HD, D: = CD/DVD drive and that’s it. They can’t imagine an environment where you have E:, F:, etc. holding data, swap space, secondary programs, etc.
The hybrid drive situation is indeed sad. I’d love a hybrid with 3TB+ of HD plus at least 64GB of fast SSD space. But the all-in-ones have too small of SSD space and the do-it-yourself separate HD+SSD w/software solutions aren’t all that great.
I don’t think you understand WinSxS. It can and will get huge. It’s essentially a backup for Windows Updates and Service Packs. And the ability to use Disk Cleanup to fix is relatively new–it came with an update to Windows 7.
I’m not sure it could make the OS balloon up to 128GB on its own, but it could easily get it halfway there.
Just got the SSD. So, I will install Windows 10 to the USB key. Someone said to boot from the USB. How does one do that?
If I install the SSD then install Windows 10 on it from the USB, will I still be able to use my current HDD with Windows 7 on it or will that cause problems?
Anything else I should know before installing the Windows 10 on the SSD? The only thing I’ve ever installed was a GPU nearly 5 years ago.