From “The New Market Wizards” by Jack Schwager, in an interview with Mark Ritchie:

How do you decide when a position is too large?
I have a rule that whenever I’m still thinking about my position when I lay my head on my pillow at night, I begin liquidation the next morning. I’m hesitant to say this because it could be misconstrued: You know that I’m a praying person. If I find myself praying about a position at any time, I liquidate it immediately. That’s a sure sign of disaster. God is not a market manipulator. I knew a trader once who thought he was. He went broke – the trader, I mean.