There are any number of devices out there running Windows 8.1 with 32 Gigs of storage and an equal number running Windows 8.1 Pro with 64 Gigs of storage.
The problem becomes more about having room for everything else (installed programs and data).... and how conscientious you are about how and where you install and save stuff.
Somewhere around 16-20GB is enough to install the OS. As Trouble mentioned above, the additional needed space depends on what programs you install and whether or not you have a large amount of data. Many of us have smallish ssds, say 60-120G that are delegated as just for the OS and programs while all data is kept on another drive.
100GB may be plenty for you but not enough for someone else. It depends on what programs you install and how much data you plan to keep.
If you place your user data into a seperate partition, then 40GB should be plenty - unless you have more than 8GB of RAM where the pagefile and hiberfile already take a large chunk - each of those files takes the same amount as the size of the RAM.
If you don't want to bother with a seperate data partition (which is highly recommended), then add the amount of data you plan to store in Documents, Music, Pictures and Videos (which can take quite a bit of space).