Start with where the idea is wrong
Identify what price behavior would make the trade idea no longer make sense. The distance to that level changes the risk in the position.
Work backward from risk
Decide what loss would be acceptable if the thesis fails, then make the position size fit that amount. Do not let a large position force a wider risk plan.
Keep uncertainty visible
A good setup can fail. Smaller, defined risk makes it easier to follow the plan rather than make emotional decisions after entry.
Intermediate takeaway
The goal is not maximum exposure. It is a position size you can manage rationally.