How should I size a position?

Position size is the amount of exposure you take, not just the amount of margin you deposit. A high-leverage position can have a much larger notional value than the cash supporting it.

Start with the amount you can lose if your invalidation level is reached. Then account for the distance to that level, fees, slippage, and the platform’s minimum size. Smaller size gives an idea more room to be wrong.

For prop-firm evaluations, size also needs to fit the daily-loss and maximum-drawdown rules. A single trade should not be able to end an account that you are trying to learn from.