Three common methods
Fixed lot — trade the same lot size every time. Simple, predictable, does not compound.
Fixed % of equity — risk a fixed percentage of current equity per trade. Auto-compounds and auto-reduces during drawdown. Most professional EA deployments use this.
Fixed fractional (Kelly-based) — allocates lot size based on the mathematical edge. Theoretically optimal but sensitive to estimation error; in practice, traders use half-Kelly to reduce variance.
MT5 EA implementation note
EAs configured with fixed lot sizes do not adapt to account size. An EA backtested on a $10,000 account at 0.10 lots should be adjusted proportionally before deploying on a $1,000 account. Failing to adjust is one of the most common deployment errors.