It's all about risk management
We have created a whole section of the site for this because it is one of the key pillars of being successful. It is no good having a winning strategy if you fail to manage your bank properly. If you bet too big you will probably eventually go bust. If you bet too small you’ll be leaving money on the table.
One of the key elements here is your attitude to risk which makes it very difficult to come up with a one size fits all approach. If you’re just starting off it is good advice to start at a low level, or even to paper trade before before upping stakes as you become comfortable with the level you are staking at. It’s much harder to drop down after starting too big and blowing a large chunk of your bankroll.
Tom goes into a fair bit of detail on Bankroll Management in Bashcast episode 186. Worth a listen if you haven’t already.