In the MCU the largest “object” we’ve seen a Flerken swallow was when Goose swallowed the 4 or 5 Kree soldiers all at the same time.
In fact the only other instance of a Flerken swallowing something was Goose with the Tesseract which is a lot smaller.
I assume the object has to get through the Flerken's throat.
Why? The tentacles and mouths are clearly bigger than the “cat” so the “throat” is either a lot bigger or the pocket dimension is inside the Flerken’s mouth. Therefore, it would go straight into the pocket dimension and not bother about any size limitations. If the Flerken can easily eat 4 Kree soldiers at the same time which is magnitudes bigger than itself then there doesn’t appear to really be a size limitation to what they can swallow.