Your broker makes money on commissions and spreads. They want you to trade volume . They will happily let you blow up your account by over-levering. They do not offer a "position sizing" button because their incentive is not aligned with your survival.
Where:
The Kelly Criterion tells you exactly how much to bet based on your win rate and risk/reward ratio. The Definitive Guide To Position Sizing Free
You want to buy at $150 . Your technical analysis suggests a stop-loss at $145 . Your broker makes money on commissions and spreads