Puppy is the smallest, likely.
Nope! At least I don't think so? I could be wrong...
While this veers off-topic, there are some very tiny Linux distros out there. Tiny Core can be had for a mere 17-ish MB. There are still several that will fit on a 1.44 MB floppy disk. They're usually 'fit for purpose' and not 'general use' distros, but they're out there.
Of all of those, I believe Tiny Core is the only one that comes with a GUI.
I did not look long. The smallest that I saw (with just one click) is 44 MB. (They have other downloads available.)
The last time I checked on the Puppies, they were getting pretty fat. Google says they vary between 300 MB and 800 MB. The largest of those won't even fit on a regular CD.
I could be wrong, and there could be an even smaller Puppy.
This is also a bit of a digression, though OP might find it interesting.
Come to think of it, I believe we have/had a forum user who used Tiny Core as their desktop.
Ah well...
I decided to dig deeper and started it in a VM. It's amazingly fast, even in a VM.

