Simple question not a simple answer. The "optimal" part is what makes the answer not so simple. To get optimal you have a lot of information to consider and a large portion will be related to your experience. An example but not limited to: trading strategy, currency pair, fundamentals, personality and on and on. The link below is how to calculate a stop loss. I would start with that first. It's what I used when I started. It will keep you out of trouble.