Position size is the amount of exposure held in a trade. Depending on the instrument, it may be shown as units of an asset, a number of contracts or a notional value. The unit must be known before the number can be interpreted.
Quantity and margin answer different questions
For a simple linear contract, notional value is underlying quantity multiplied by the relevant price. If quantity is displayed as contracts, the contract size or multiplier must also be included where required. An inverse contract may instead specify a fixed quote-currency face value per contract.
Margin is the collateral supporting exposure, not the position quantity itself. Two positions with identical size can use different margin amounts or leverage settings. For unchanged linear exposure and the same price movement, changing leverage alone does not change gross PnL.
Size connects a trade to its risk
A larger position magnifies the money gained or lost from a given price move. It can also require more liquidity to exit and encounter different margin tiers or position limits.
Sizing therefore involves more than choosing a leverage number: stop distance, execution costs, collateral and other correlated positions also matter. Closing only part of a position reduces its size while leaving the remainder exposed.