I was going to write a simple "me too:" I'm also a bit annoyed that there isn't (as far as I can tell) a function for getting the number of slots on a robot's own inventory.
The documentation says that the own inventory is accessed with sides.back but that returns an "invalid side" error.
My guess is that the "historic" ability to use getStackInSlot() for the robot's internal API went from "deprecated" to "scrapped" but the author forgot to provide a replacement for getInventorySize() .
Anyway, happily for both of us, there's a workaround: The function getStackInInternalSlot() e
