Backtest offers 3 capital management tools. One that will always calculate order size as initial capital/close, one that uses the same calculation but adds or substracts profit. The last one allows you to enter manually the size you want to trade.
The time-based stop will close the position after a set number of bars.
In true TradingView spirit, the author of this script has published it open-source, so traders can understand and verify it. Cheers to the author! You may use it for free, but reuse of this code in a publication is governed by House Rules. You can favorite it to use it on a chart.