I am using a 240GB SSD and I have Windows installed along with MS Office, Adobe CS6, FFXI, FFXIV, ANNO 2770, MS Flight, MS Flight X and a ton of other programs and games and I have around 100GB left. What I would do is see how much space your currently using on your C drive and kinda go with that and give yourself some room to grow, I think if you got anything in the 200GB+ range you will be fine.
I also dont know what size to get I will only put os and possibly games on it because I have my hitachui deskstars...would 120 gb be enough for win 7 pro 64 bit and like 10 games???
My Win 7 Pro 64 eats about 28gb. Once you start adding drivers, browsers,anti-virus, codecs, plug-ins, office, utilities and all that jazz, it's easy to see it start going up. To be conservative, expect all that stuff to eat about 50gbs. After that, just add up some of the games you would like to install. My WoW install eats 28gbs (and loads only slightly faster on the SSD). My SW:TOR install eats 20gbs. Shogun 2 eats 25gbs (MUCH faster loading on the SSD). Some games simply don't need the speed of the SSD (Starcraft II loads only slightly faster - so it's installed on my HDDs instead).
It's really a matter of balancing the demanding apps where there is lots of data (particularly random reads) to load vs. the less-demanding titles which don't see much benefit. 120gb has been the usual recommendation but as mentioned above, a 240gb pretty well covers most people's app needs with some room to spare. Having a smaller SSD with Steam installed on it can be annoying when you have to 'backup' or uninstall games because your volume is low on space and you want a new game. If your budget will allow it, and you want the least future hassles, get a 240gb.