Wikipedia agrees (https://en.wikipedia.org/wiki/History_of_the_floppy_disk):

8-inch: 1972

5.25-inch: 1976

3.5-inch: 1982

The IBM 23FD (read-only) was released for the System 370 in 1971. The first read-write floppy drive, Memorex 650, was available in 1972.

https://www.computerhistory.org/timeline/1968/

The first time i saw an 8-inch disk, i thought it was some sort of wind-up. But no, there really were disks that big once; my dad had one from some ancient minicomputer he'd worked on at some point.

Father worked at DEC; picked up a PDT-11/150 on discount as a home system. Intended by DEC as an industrial controller.

Low-level LSI-11 processor variant, 128k RAM, (2) 8" floppy drives. The case was 75% floppy drive, 25 % processor boards on top.

Came with RT-11 build floppies, I had to run the OS build in order to generate a boot disk and three language/utility disks. Took about 8 hours to do the full optioning and build from the 13 source floppies.

Ahh, the old days. Wrote my first D&D game, ballistic path analysis, mandelbrot drawn to 80x24 greenscreen VT01...

Early in my career and after a field trip abroad commissioning an aircraft simulator I was stopped at customs with a box of them and was asked what they were worth. Said that we could send the data by modem and so effectively nothing. I was allowed to pass.

I have a stack of 8" floppies sitting on my desk. Also have the old Wang computer they go to laying around. I have not been able to find the actual disk drive though.

Back in the late 80s we had a PL/M development system from Intel we used to program 8051 and 8096 microcontrollers for industrial sensors at Honeywell. The code was saved on 8" floppies.

A photo of that bad boy http://home.kpn.nl/a.dikker1/museum/blauwebak.html

Yep, you could punch holes in them and put them into binders. Their size was just right for this. Supposedly, it was a perfect way to keep them organized.

The TRS-80 Model II used them. Some CP/M microcomputers did too.

I had an old Model II 8" drive I had pulled out of old machine that I used in the Intro to Computing class I taught in the 90's. It made a great prop to show how drives work. The read/write head was about the size of a matchbox.

And in use until recently! https://www.cnbc.com/2016/05/25/us-military-uses-8-inch-flop...