1. Learn Price TA so that you know where "support" and "resistance" are on the chart. 2. Most logical place for a stop is just below support or just above resistance. Those levels are SUPPOSED to hold if tested... by definition 3. The size of your stop should be determined by "how close to support/resistance" you got your position. If you were a bit late with your play (that is, if support/resistance is further away from your trade), you need a larger stop to be technically sensible/correct. Of course, you can always use an "arbitrary stop"... like 2 points or 50 pips. Those can also work when the market goes IMMEDIATELY in your direction with little or no "back flushing".
The size of the stop loss in each individual transaction is different for me. It depends on the currency pair I am trading, its volatility, where the nearest significant support or resistance level is located, and some other conditions of my trading strategy. Therefore, it is not possible to set the same stop loss for all instruments.
Scalping ~2-3pips with 10-15pip profit target, 1-2times per week that scalp moves 30-50pips, 1-2times per month that scalp moves 100-200pips, most people use 0.5% to 2% of account balance per trade, never had much respect for that, using a multiple of timeframe ATR gives you a good starting point.