The formula
Size = (capital × risk %) ÷ (distance between entry and stop × point value). Risk is how much you lose if price hits the stop: decide that first, and the size follows from it.
Crypto example
Capital €10,000, risk 1% = €100. BTC entry at 64,000, stop at 63,500: distance 500. Size = 100 ÷ 500 = 0.2 BTC. If the stop is hit you lose €100, no more.
Forex and futures example
In forex a standard lot is 100,000 units: on EUR/USD one pip (0.0001) is worth about $10 per lot. With $100 of risk and a 20-pip stop, you open 0.5 lots.
In futures every contract has a point value: the E-mini S&P 500 is worth $50 per point. With $500 of risk and a 5-point stop you open 2 contracts.
Mistakes to avoid
Choosing the size based on how much you want to make, increasing it after a loss to win it back, forgetting fees and funding on longer positions.