Your lot size is the money you are prepared to lose divided by what the stop costs for one lot. The money at risk is the account size multiplied by your risk percentage; the cost per lot is the stop in pips multiplied by the pip value of one lot. Sizing this way keeps every loss the same size no matter how wide the stop is, which is what lets a strategy survive a losing streak. Work out the pip value first with the pip calculator, then check where the resulting position lands against your exposure cap with the risk calculator.