Account Risk
Before you can use appropriate position sizing for a specific trade, you must determine the account risk. This typically gets expressed as a percentage of your capital. As a rule of thumb, most retail investors risk no more than 2% of their investment capital on any one trade; fund managers usually risk less than this amount.
For example, if a trader has a $25,000 account and decides to set their maximum account risk at 2%, they cannot risk more than $500 per trade (2% x $25,000). Even if he loses 10 consecutive trades in a row, he will only loose 20% of the trading capital.