I know that "back in the day" it was the case of buying the biggest HDD you could afford, but these days it is more important to have a good processor and a reasonable amount of RAM.
So, with that in mind how big a HDD do I really need? I would be buying a PC with Windows 7 (may upgrade to 10 in the future) and at least 4GB of RAM. I use Onedrive for storage of documents/storage and do not store anything else on the actual hard drive other than maybe 100 photographs maximum.
For example the HP ProBook has a 111GB HDD and of that I have used 26.5GB which includes the OS and two user accounts.
What I want to know is the minimum free space required to run Windows 7 (and in the future 10) smoothly.