Risk comes before size
Choose the maximum acceptable loss first. Position size is then derived from that amount, the distance to invalidation and the instrument’s value per price movement.
Account for real trading costs
Spread, commission, slippage and gaps can make realised loss larger than the planned distance. Historical testing should include these costs where possible.
Survival is the first constraint
No setup is certain. Limits on per-trade risk, correlated exposure and total drawdown help keep a sequence of ordinary losses from becoming unrecoverable.