Don
I run a 256GB PCI-E SSD for my OS and a 512 GB for steam and other games.
256 seems good for the most part prior to the pci-e ssd I split my 512GB into 2 for windows and games.
Games on steam are reaching 60GB so 120 is out of the question due to how steam un packs. Unless Valve change that then 256 will be too small in a few years.
I purge off old games that I'm not currently playing. If I fancy playing one, then I'll just leave it trickling down whilst I'm playing another game