This passage is in the Book of Daniel, which does not say. Since the historical evidence strongly suggests that the Book of Daniel was a second-century BCE work of fiction, there is no point speculating on the size of the lion's den.
The captivity of Israel started about 583 BC.Daniel 1:21 says that he continued until the reign of Cyrus. Daniel 10:1 mentions the third year of Cyrus.Cyrus overtook Babylon around 539 BC.So Daniel...
He was in there a little more than one night - plenty of time for the lions to eat him if God had let them as they were pretty hungry as we can see when we read the story, because Daniel's accusers...