The position sizing tool is an indicator to help calculate in trading, such as loss and gain, lots size, and risk-reward ratio.
When you open the indicator, you must select the entry, take profit, and stop-loss points.
Be careful;
The take profit point must be more than the entry point in the long position. On the other hand, it will be a short position.
The stop loss point must be less than the entry point in the long position. On the other hand, it will be a short position.
For contract size, you can find details on MetaTrader, Ctrader, or your broker.