Back in the day ... there were single-board PCs that would actually fit in a half-height 5 1/4" floppy bay. Saw some at the Computer Faire's bazaar while searching for a pair of floppies and a case to add on to my Radio Shack Model 1. A two-drive floppy case could hold the computer and a floppy - enough to work with. A 4-drive (half-height, equivalent to 2 full-height floppies) case could hold 2 floppies or one and a hard disk (mostly 10-20MB at the time) along with the computer. Not popular with hobbyists, but perhaps industrial. Simply the fact that you could get a complete computer on a board that size was amazing at the time.
"Desktop hard drive sized" might have been a better description...But that sure doesn't sound as good. Many younger readers are probably as familiarized to a floppy drive as the average latter-day 3.5in floppy users to punched tapes and cards.
The logical next step would be a single board computer the size, including thickness, of an actual 3.5in floppy.