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.
RATCHX3 High Jinks vom Neuanfang - Gotchya Day: 1/23/2013 DN, SG, SR, CA
Inspire Spark to Ignition! (Ignite) Gotchya Day: 10/8/2016
C-ATCH Pimg - RATO, UJJ (x2), DJX, HIT, CA, CGC, High in Games, Reserve High in Standard- CPE Nationals 2014