Where should I place my stop loss?

At the price where your trade idea is objectively wrong — not at a fixed percent chosen for comfort. For trend entries that's typically beyond the recent swing low/high; for volatility-aware placement, a multiple of ATR (commonly 1.5–3×) adapts the distance to how much the market normally wiggles, so normal noise doesn't stop you out of a good position.

Stops interact with position size: a wider stop isn't riskier if you shrink the position to keep risk-per-trade constant. The combination worth testing (and what a backtest settles empirically per strategy) is stop distance × position size × whether a trailing mechanism improves or hurts that specific system.