HDD size depends on what you want to do with it and how long you might keep it.
If you just want a word processor 8.4 GB would probably be fine. If you expect to use it for some time a larger HD would help proof against larger software files. If you want computer video, then much, much larger.
I am sure someone will say the largest you can afford. Don't forget you can always move the HDD to a newer computer.
Hope this helps.