60 GB is plenty big enough for your OS drive, even when ALL apps are installed there. I have Win 7 Ultimate 64 Bit, Office 2010 (32 Bit) full install and quite a few other apps and only use about 25 GB of my OS partition. I would advise to move date (using the approved method) to the separate HD. I would be surprised if you use mor than half the SSD.
You are correct. The bare-bones Win7 OS is around 30, give or take; x86, slightly under & x64 slightly over. 60 is enough, just store data elsewhere & there is still room for customary programs, like, as mentioned, Office or whatever.
I have a "barebones" Ultimate (64) installation on one of my laptops, purely for comparison purposes. It takes up just under 14Gbs. In use, making allowance for hibernation files, increasing prefetch size and pagefile use, it quickly swells to 18/19 Gbs.
But that, of course, is only a part of the story. Allwance has to be made for your own programs, possuble extended use of homegroups and add-ons/saved requiements of your installed programs.
Unless you are a dedicated game player, which can eat up disk space, I would also agree that 60 Gbs would be ample under most circumstances.