Great question, Maverick. There are two things that I look out to tell me that I've gotten too big. The first is my theta. My ideal number is equal to 1/10 of 1% of my net liq. My range for this $150k account is between 100 and 150 theta per day.
The second way that I know that my account is too big involves a look at the analyze tab. I stress the account for a 20% down move with a 30 point increase in volatility. I also stress to the upside by 15%. As I look now a 20% down move would pretty much wipe me out. I got in this position because I've been trading on my iPhone and I can't see the analyze tab. On the rare dates that I am trading from my laptop I check to see where I'm at. So for now, I'm out of ES and into spy until some of my ES positions close out.