Account Portion Per Trade Position

To manage your invested fund well, you have to decide before the opening of any position how much of the money you can afford to lose in case the trade goes negative from your projection. For instance, you may decide that for every opened position your risked money will be 3%, 5% or 10% of the total fund, by so doing you have known prior to the execution of the trade the highest amount that can ever go out of your money on that single trading position, by so doing you have even taken away emotion.
The factor needed to work out this are:

1. The fund balance in your account.
2. The number of pip set as stop loss.
3. The lot size (volume) traded.

For example:
Let's say your fund balance is $5000 and your predetermined stop loss pip is 50 pips (selecting the number of your stop-loss pips should be from your analytical research) and you are ready to risk only 2% of your fund for a position.
What do you do?
Work out the 2% of $5000
Which is = $100.
Implying that you can afford to lose $100 in case of any eventuality.
Then, Divide $100 by 50 pips
It will be $2
Your lot size must be 1 pip to $2. That will be 0.2 lot size.
So you must use 0.2 lot size.
As much as possible try not to be greedy, to be less greedy is to be able to minimize risk.
In a way leverage can help to control risk: if your leverage is relatively low it will limit you against opening a trade with high lot size.