Do devices on WiFi eat Data Operations?
Yes, any Particle.publish for both Wi-Fi and cellular.
Do Webhook actions count as Data Operations?
The webhook itself does not, but the Particle.publish that triggered it does.
If you are receiving data using the hook response, that will use data operations.
Do API/Web Console, Events & User Initiated Health Checks count?
Using the console or API itself does not use data operations. Calling a function or variable directly from the console does. Fleet health does not.
Is there a size limit for a single “Data Operation”?
622 bytes. With device OS 3.0-rc.2 and later it is 1024 bytes on some devices.
https://docs.particle.io/tutorials/device-cloud/introduction/#pricing