I think you are taking the wrong approach. Don't go into live trading yet. Keep on demo until you can answer that question yourself.
To be more clear: a s/l should be a function of your expectancy, if you know, for example, that your system can produce 50% winners. has a 2:1 risk reward and normally has strings of loses of 3 - 5 then you could easily determine how much you can "afford" to lose in each trade not to put your account at risk.
But from the nature of your question I can asume you are still far from really knowing and understanding the possibilites of your system. I'd say there's no need to risk real capital, even if you can afford to lose it all, for now.