Too many traders focus only on the entry price and pay insufficient attention to the size of the position. Trading too large can result in good trades being liquidated at a loss because of fear.
On the other hand, trading larger than normal when the profit potential appears to be much greater than the risk is one of the key ways in which many of the Market Wizards achieve superior returns. Trading smaller, or not at all, for lower probability trades and larger for higher probability trades can even transform a losing strategy into a winning one.
For example, Edward Thorp, who started out devising strategies to win at casino games before achieving an extraordinary return/risk record as a hedge fund manager, discovered that by varying the bet size based on perceived probabilities, he could transform the negative edge in Blackjack into a positive edge. An analogous principal would apply to a trading strategy in which it was possible to identify higher and lower probability trades.