128Gb is probably sufficient for an OS SSD, however- I think it's kind of pushing it in my opinion. I made the mistake of putting a 64Gb SSD in my machine as an OS drive for Win7. Within a couple months, I can no longer boot it because windoze updates have completely filled the drive. So like I say, 128Gb is probably sufficient, but if I were to do that again I'd be looking at AT LEAST a 256Gb SSD for OS purposes.
C-ATCH Pimg - DOB: 2/06, RATO, UJJ (x2), DJA, HIT, CA, CGC, High in Games, Reserve High in Standard- CPE Nationals 2014
RATCHX2 High Jinks vom Neuanfang - DOB 9/12 (Gotchya Day: 1/23/2013) DN, CA, agility superstar in training