120 GB minimum. Any smaller, and you will feel squeezed before too long.
Applications don't take up that much space individually, but you want some breathing room.
My 128GB Kingston has Win 8 Pro, and all the applications on it besides games.
Office 2013, Adobe Lightroom, Corel Video Studio and PaintShop Pro, etc etc.
Currently ~50GB used space. A 64GB would pretty much zero room for growth with my usage.