O tamanho da criatura não importa
DMG, página 153:
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. A creature inside the bag can use its action to try to escape with a successful DC 15 Strength check. Another creature can use its action to reach into the bag to pull a creature out, doing so with a successful DC 20 Strength check (provided it isn't pulled inside the bag first). Any creature that starts its turn inside the bag is devoured, its body destroyed.
Não há palavras sobre o tamanho da criatura. Então RAW não importa.
É um item maravilhoso, amigo, medições padrão aqui não são regras:)