Page 153 DMG:
Bag of Devouring
This bag superficially resembles a Bag of holding.
Inanimate objects can be stored in the bag, which can hold a cubic foot of such material.
When part of a living creature is placed in the bag, as happens when someone reaches inside it, there is a 50 percent chance that the creature is pulled inside the bag.
Bag of Holding
This bag has an interior space considerably larger than its outside dimensions, roughly 2 feet in diameter at the mouth and 4 feet deep.
As the bag superficially resembles a Bag of Holding, that means it is 2 feet in diameter at the mouth and 4 feet deep. Internally, it is explicitly stated that it holds up to 1 cubic foot of inanimate objects.
Given these measurements, what is its internal storage capacity for living creatures that are pulled into the bag, and how does its 2-foot diameter mouth change (if at all) depending on creature size?
A Medium sized creature could fit through a 2-foot opening, but what happens when a Gargantuan Brontosaurus sticks its head into it?