This general question is like asking what s[censored] size you should use - it depends entirely on personal circumstances.

Unless you are fortunate enough to have a rich daddy, most of us either have to limit our losses, or go back to waiting on tables. Position size is a way to control your exposure to market risk - it pays to see yourself as a private insurance company buying up risk.

If your personal risk tolerance is say, 30% drawdowns (DD), and your backtest results show historical DDs of 60%, then your position size (for that particular fx pair with that particular strategy) should be decreased to 50%, halving your risk exposure to 30%, as well as your potential profits. Conversely, if your backtest results show a max DD of 10%, then your pos size should be increased to 300% (x3 leverage, tripling your profits).

All the above assumes that you have:

1) a strategy with clear entry and exit rules;
2) the necessary tools to determine profitability and risk (e.g., VST);
3) sufficient trading capital;
4) the necessary cojones to stick to your strategy.