It’s not easy to say precisely what makes a question big; but we can at least give a few examples from the history of philosophy so that we have some idea what we’re talking about:
- What is the meaning of life?
- What is the nature of ultimate reality?
- What is Being?
- Is there a god?
- Is there some sort of cosmic justice?
- What is the self ?
- Does a person’s self (mind, soul) persist after death?
- Do we have free will?
- Why be moral?
- What is the good life for a human being?
- What are the foundations of our knowledge?
- What are the limits to what we can know?
- What is truth?
- What is the good?
- What is justice?
- What is virtue?
- What is beauty?
- What is life?
- Why is there something rather than nothing?
More here.