The relevant mechanics are the same as for a bag of holding.
Size of Interior: "Gigantic"
The bag of devouring's description says:
This bag superficially resembles a bag of holding but is a feeding orifice for a gigantic extradimensional creature.
Gigantic isn't one of the official creature sizes (which are Tiny, Small, Medium, Large, Huge, and Gargantuan), but plain language tells us it's big. Therefore the extradimensional creature is indeterminately big, so it might well be big enough to ingest any creature that has a stat block.
Size of Opening: Roughly 2 Feet in Diameter
However, the size of the Eater is less relevant to the mechanics than the size of the bag's opening. The bag of holding's description says its interior space is:
...roughly 2 feet in diameter at the mouth...
By specifying "at the mouth," this is implying that the bag of holding has an opening that is or expands to the size of its smallest interior dimension, a 2-foot diameter. (Such expansion need not be magical. For example, picture the opening of an accordion-style folder.)
You could extrapolate that the mouth of the bag of devouring is also scaled to the size of its internal space, but that space is indeterminately large so this is impossible for a physical bag. Instead, remember that the bag of devouring's description says, "This bag superficially resembles a bag of holding..." The whole point is it's camouflaged to look like a bag of holding, so its opening will have the same 2-foot diameter. Therefore no Gargantuan or Huge creature is going to fit, and probably no Large one either.
Of course, no one's going to sit idly by while you shove them into your bag, regardless of whether it's a bag of devouring or a bag of holding (since the latter will kill you just as dead, by suffocation). You'll have to overcome their resistance before you can even get to the 50% chance of ingestion, DC15 Str check to attempt to escape the bag (or DC20 to attempt to pull someone out), and, ultimately, them being devoured and their body destroyed.