Position size is a derived quantity. Once you have decided how much of your account you are prepared to lose if an idea fails, and you know how far away the invalidation level sits, the size follows arithmetically.
The relationship
Risk amount equals account equity multiplied by the fraction of equity you are prepared to risk. Position size equals that risk amount divided by the distance to invalidation, expressed in the value per unit of price movement for the instrument. Widening the stop without reducing size increases the amount at risk proportionally.
Why the order of operations matters
Traders frequently choose a size first and then place the invalidation wherever it looks comfortable on the chart. This inverts the logic: the level should reflect where the idea is wrong, and the size should then be reduced until the resulting loss is acceptable. If the acceptable size becomes impractically small, the correct conclusion is usually that the trade is not viable at that account size.
Leverage is not risk
Leverage determines the margin required to hold a position. It does not determine how much you can lose — the distance to invalidation and the size do. High leverage makes large positions possible; it is the position size that makes losses large.
Costs and gaps
A stop-loss order is an instruction, not a guarantee of price. In gapping or illiquid conditions an order can execute beyond the requested level, producing a larger loss than the calculation assumed. Guaranteed stop products exist at some providers and typically carry a premium; their availability and terms differ by provider and jurisdiction.
Drawdown compounds
Losses require larger percentage gains to recover than the percentage lost, and the gap widens as drawdown deepens. This asymmetry is the practical argument for keeping per-trade risk small enough that a normal losing sequence remains survivable.