There's a bunch of nostalgia about what small meant to mean, so I'll leave some links to the QNX 1.44MB demo floppy:
My first "real OS" experience was OS-9, which IIRC booted from a 400KB floppy. I believe Radio Shack sold it on a cartridge as well.
Talk about an OS that should have utterly destroyed DOS and all other CLI-based OSs.
I first encountered it at an oil-terminal company I worked at in the 80's and couldn't believe the features packed into such a small footprint.
It's real-time event handling was top-notch, and it was stable as all-hell...in a sane world it would have dominated the market.