250GB should be plenty. Usual OS and apps shouldn't take more than 50GB. And each game takes about 7GB a piece. I just upgraded from a 128 to 256GB; 128 is just large enough, but I wanted room to grow.
You could always put in a large HDD as a secondary drive for your iTunes library, photos, and videos that don't require SSD speeds.
SSD = OS + programs + most used games.
... Typical OS + programs take around 30-> 35 gigs, Using most common tweaks. Add to that most often used games. For SSDs Must leave a MIN of 15% free so that Trim, CG, and wear leveling can work their magic to keep SSD performing optimumly.
.. HDD. Place itune's here (for My wifes system I placed a small partition just for itunes). Place your BACKup System image here - Done from control panel, backup as this comes in VERY handy if you ever need to re-install OS. Also for all downloads, and files you generate.
For calculation: ie for 128 gig SSD. Formated size is 119 Gigs, then subtract 18 gigs, so usable space for a 128 gig SSD is approx 101 gigs. For a 256 gig that works out to about 204 Gigs usable.