I have fit Windows 10 64-bit onto a 40GB SSD with 17GB free space, but nothing else was installed.
I would suggest starting with a drive that is at least 240GB in size, however if your not planning on installing software and games to it then a 128GB would be fine. You could get by with less storage capacity, but you dont want to have to add USB sticks with symbolic linking later to hack your way to a system with greater capacity when you can just get a drive that is plenty big enough and some room for current and future data storage and software installation needs.
One risk I face with my wifes computer running on this older 40GB SSD is that as Windows 10 updates sometimes it requires 16GB free in order to update. I have a Intel Atom Tablet that has 32GB internal storage and it complains that Windows 10 is out of date and on that device the most I am able to free up is 14GB. So in order to update it I may have to perform a trick with a MicroSD card and setting up a symbolic link to the SD Cards storage to span the internal storage to that to make the C: drive look to have more free space with the 32GB Micro SD Card added and symbolic link created for the Microsoft update target location.
Yesterday my wife wanted to install World of Warcraft to her system because she saw me playing it again and WoW is 66GB in size. I ended up installing a 320GB Hard Drive from a laptop into the desktop computer after copying my install of WoW to this HDD with my USB SATA HDD Dock before installation into her computer to avoid having to wait for the download, then point battlenet to this E: drive WoW location.
