Many traders start with a rule: risk only a small fraction of equity on a single idea. Position sizing translates that rule into lots or units.
A practical workflow is: decide maximum risk in currency, measure stop distance in pips or price units, then solve for size so that if the stop is hit, loss stays inside your cap. Different brokers expose contract specifications differently, so verify pip value on your symbol.
Past examples do not guarantee future results. Leverage magnifies both gains and losses.