Risking a fixed percentage of your account on every trade - rather than a fixed dollar amount or a fixed number of units - is what keeps a string of losses from compounding into an account-ending one. Fill in your account size, how much of it you're willing to risk on this trade, and the stop distance (or pick a live signal to auto-fill it).
Rule of thumb, not a guarantee - actual contract/lot sizing varies by instrument and broker. Verify against your broker's own contract specs before sizing a real position.