The size that gives you the best ROI. Depends on many factors, so is different for every person. Depends on volatility, size versus market volume, leverage, margin requirements, drawdown, time to recover...
I don't like this. If you have 1 million then of course you can afford 1 million. Of course. Of course, you can blow up one million.
I had to experience the negative aspects of money management and risk management in order to create and maintain the right attitude toward profitability. For example, the number of shares I was going for was linked to my trading capital. I did not realize that the number of shares played should be linked to acceptable risk. Today I always have my risk defined, and I trade according to it. If, for instance, risk is defined as a $250 loss per trade and based on the way the stock trade shows that you can keep a 50 cent stop with confidence, then my share size on this stock is 500 shares. - Techniques of Tape reading
Not really. I can't afford to lose my trading capital. I usually risk less than 1% of that capital on any one trade. I don't see a scenario where I would lose all my capital. the optimum capital for day trading depends on what you expect your results to be, how much risk you are willing to take, and what type of trader you are. If you expect to make a 100k a year trading I don't think 100K account size is enough. In that case you are right, you better be able to afford to lose your capital.
Supposedly, if a trader wants to earn 100K per year, he has to have a trading account between 1 million and 10 million dollars. https://blog.darwinex.com/trading-capital-to-earn-100k-a-year/ https://bclund.com/2013/06/02/so-you-want-to-trade-for-a-living-how-much-money-do-you-really-need/
There is no comparison between day trading strategies with little capital like $100k and investment strategies with billions like Soros, Buffet, Icahn, etc. Little capital have more options to grow faster, 20%/year is nothing.