Hello,
Choosing a SSD to hold Win-7 OS, and a few programs, you will need 128GB. That's actually larger than bare minimum but you need space for the pagefile, and about 25% for over provisioning. The ideal size is 256GB where you have plenty of room for your programs, My documents, and other files, and down the road a year or so, you won't have to begin looking for another one as your primary SSD becomes filled.
Also when you look at the specs, the 256GB SSD's are significantly faster than the smaller 60GB and 128GB ones.
Prices now are below $1/GB, which is very good.
So, my vote is 256GB!