Position sizing is a technique used in forex trading to manage the size of a trading position relative to the available trading capital and the risk involved in the trade. It is an essential aspect of risk management in forex trading as it helps traders to manage their risk exposure while maximizing their profits.

1. Fixed Position Size: This is a position sizing technique where traders trade with a fixed number of lots or units, regardless of the trading capital and the size of the stop loss. The main disadvantage of this technique is that it does not consider the risk involved in the trade, and it can lead to significant losses if the market moves against the trader.
2. Percentage Risk: This position sizing technique is based on the percentage of the trading capital that a trader is willing to risk on a trade. For example, a trader who has $10,000 in trading capital and is willing to risk 2% on a trade would risk $200 on that trade. The advantage of this technique is that it allows traders to manage their risk exposure based on the size of their trading capital.
3. Volatility-Based Position Sizing: This position sizing technique is based on the volatility of the currency pair being traded. It involves adjusting the position size based on the expected volatility of the market, which is usually measured by the Average True Range (ATR) indicator. This technique ensures that traders are not risking more than they should based on the volatility of the market.
4. Kelly Criterion: This position sizing technique is based on a mathematical formula that calculates the optimal position size based on the probability of winning or losing a trade. The formula takes into account the size of the trading capital, the risk-reward ratio, and the probability of winning or losing a trade.