Maybe use some formula. In all input fields you can write MQL4 code. You can use functions, +, -, * and /. But what would be the formula, I don't exactly know. I think you will also need to use at least one Variable.
It also depends whether you want to use hard Stop-Loss/Take-Profit or monitor the profit and close the trade when certain profit/loss is reached. Different blocks are used in these cases. And if you use hard stops , there is something that we can call Tick value, which is the amount of money (in your account currency) that your trade would profit/lose if the price moves with 1 tick. This is little bit complicated... at least for me.
Otherwise there is no such option that turns lot size into SL/TP size, and I don't even remember someone asking for this 🙂 And I don't know how good is the idea. Because even if you put let's say 10 pips normal SL, the loss would be proportional to the lot size anyway.