Size SSD For OS Only?

The operating system itself uses no more than 20 to 30 GB's. Allowing for some growth and a few programs, I would say a 128 GB SSD should be plenty. You could probably use a smaller one, but might run into issues in the future. An SSD makes a huge improvement to the responsiveness of the system. I think it's one of the best upgrades you can make. Depending on how much you have on your current drive, a larger SSD might make sense. I keep everything on mine and use an HDD solely for storing backups.
 

USAFRet

Titan
Moderator
Size depends on the OS.
A Win 7 64bit system will consume ~45GB, once all the current updates are run.
A Win 8.1 system, ~35GB.
A Win 10 system can fit within a 30GB drive.

All of these will increase as time goes on.
And then you add any applications. It's never 'just the OS'.

Up until very recently, a 120GB SSD was my primary drive. Prices today makes that 120GB a non-starter for a new buy.