Position size is the amount you are willing to risk divided by the distance between your entry and your stop. Because it works in price rather than pips, the same calculation covers every instrument: a fifty-point stop on an index and a fifty-pip stop on a pair both resolve to a size that risks the same money. A tighter stop allows a larger position for the same risk, and a wider one demands a smaller position, which is exactly the trade-off you want the maths to make for you rather than making it by feel. Cross-check the result against your exposure cap with the risk calculator.