On position sizing vs. risk per trade

Hey everyone, been lurking for a bit, figured I'd jump in with a quick question. I'm still trying to nail down consistent profitability, and one area I seem to struggle with is really understanding the interplay between position sizing and managing risk per trade. I get the theory – don't risk more than X% of your account – but in practice, especially with varying volatility and different asset classes (say, $EURUSD vs. a more volatile microcap stock), my sizing often feels like a guess. Are most of you adjusting your position size based on ATR or volatility for every trade to maintain a fixed dollar risk, or is it more about a general percentage based on your stop-loss distance for a given instrument? How do you practically implement that adjustment without overcomplicating things on the fly?