Those were referred to as "floppy disks" (although they weren't very floppy!) or diskettes. They were measured in inches.
Be sure to scroll down to the chart showing the different sizes, years used, formats, etc.
Drawings of the 5.25" and 3.5" are shown here, along with explanations.
My first two computers ran only on 5.25" diskettes -- program on one, data on the other. No hard drive.
Then my son installed a hard drive in place of one of the 5.25" drives, so I learned to put programs on the hard drive and use the diskettes for data.
Then he replaced the 5.25" with a 3.5" diskette -- the smaller ones held LOTS more data than the larger one did!
My next several computers had hard drives and one A drive (for 3.5" diskettes). One of them also had a zip drive -- http://www.webopedia.com/TERM/Z/ZIP.html -- for zip disks which held an amazing amount of data. I used those only for storage of photos!!
Now everything is on the hard drive, and I back data up to the separate back-up drive, which is simply an external hard drive. No more little diskettes. I use email attachments or "flash drives" if I want to transfer data from one computer to another.
Times do change.
8" drives were often for specifc instruments or application rather than for a PC. The computers attached to Mass spectrometers and ICPOES (a type of optical emission spectrometer) were two I came across that used these in the 1980s. They look like a scaled up version of a 5 1/4" disk.