I got an 120GB SSD for my OS- Windows 10 takes up less than 10GB, of course over time it will start collecting temp files, and other crap, which, if not regularly cleaned, will start taking up a considerable amount of storage. But I keep a couple of programs on that drive as well, plus all that stuff that came with MoBo and GPU... And I'm far from maxing the capacity on that drive.
Then, I got 1TB HDD for my games, I got my Steam, Origin, Uplay and GOG here, with all games installed at all times, plus some files I download from the web. And I'm actually thinking about getting another 1TB drive, to be honest with you, all the big titles weight around 50-70GBs nowadays, with some heading towards 100GB...
In your case, I would partition your drive 100GB-400GB, install ONLY Windows on the 100GB partition, and keep your games on the other. Also, that way if your Windows ever needs to be reinstalled, you only format the system partition.