Most trading losses that end accounts are not bad ideas; they are good ideas sized badly. Position sizing is the habit of choosing how much to trade from how much you can afford to lose, rather than from how confident you feel.
Start with a risk per trade, commonly one or two percent of the account. On a 5,000 dollar account at one percent, the maximum loss on any single trade is 50 dollars.
Next, measure the stop. If your analysis says the trade is wrong once price moves 25 pips against you, the stop is 25 pips. Do not move the stop to fit a position size; adjust the size to fit the stop.
Divide the risk by the stop distance in pip value terms. Fifty dollars divided by 25 pips is 2 dollars per pip, which on EUR/USD is 0.2 lots. That is the trade size, regardless of the leverage available.
Sizing this way makes every trade the same risk, so a run of losses is survivable and a run of wins compounds cleanly. The position size calculator does the arithmetic for any pair and account currency.