Size from the cash you can lose to the stop — not from how exciting the setup looks.
The core formula
Choose a max account risk per idea (for example 0.25%–1%). Measure stop distance in price. Solve for position size so that a stop-out loses about that cash amount. Leverage then becomes an output, not an input.
Volatility changes the stop
ATR-based or structure-based stops should widen in wild regimes. If you keep size fixed while stops widen, you are secretly raising account risk. Recalculate.
Correlated ideas share a budget
Three USD shorts are often one risk. Cap total correlated exposure, not only per-ticket risk.
Platform translation
Lot sizes, contract multipliers, and mini/micro symbols differ. Verify the cash risk on a calculator or demo before going live with a new instrument.