I always think in terms of $100,000 blocks. For each trade, I never risk more than 1% of my capital. That is how all my trading is conducted. 
I had my 100K trading blocks go from 100K to 160K last year. In the beginning of the year, I had a series of losers. The markets were choppy. This cost me nearly 40K per 100K block. As my account size decreases, so does my risk. This is called the Ritchie Rule.
Risk management, risk management and more risk management.