I don't believe you can do that.
The trade panel is the trade panel, and position size in a Strategy is position size in a Strategy.
Perfectly possible to change the position size based on pre-calculated SL or other factors (buy or sell, time of day, Profit/Loss already gained/suffered during trading day, ...) to take risk appetite into account, but I have never seen the possibility to change the value of the trade panel on screen.
Anyone who has, perhaps ?