I use 128MB, anything less can cause trouble.
I did test down to 96MB but 128MB seemed safest, as these things run 24/7/365.
Also depends on the OS, Pikrellcam is Raspbian ? Lite
.MotionEyeOS is Buildroot, I tinker with piCore and Ultibo, OS space gets smaller each time so less of an issue.
The 1GB Pi's I never worry about they seem to have plenty of room.
Zero W is a bit of a pain as the wireless works best with Lite which takes the most memory.
Ideally Pikrellcam on piCore would be my preferred solution, but piCore is not Debian based so I need a higher skillset to port it
Hopefully new kernel releases with solve some issues, min 4.9?
A smaller motion camera solution would be great as I have a Clusterhat and and I'm slowly acquiring more normal Zeros to populate it with 4 Pi's.
Pikrellcam will run on the mother Pi 2 and trigger an smaller app on the Zeros to capture at the same time.
USB booting the Zero's on the hat is doable, loading a full LIte + Pikrellcam , very, very slow progress
I may just have to figure out how to roll my own Debian OS, pretty sure I don't need all 1.3GB+ of Raspbian Lite.
Especially when I know i can get raspivid/still working on piCore <100MB. Even old 256MB A's should work?
I'm dancing on Rainbows.
Raspberries are not Apples or Oranges