After using a default order size of 100 for 2 years I finally changed it to 500. This did wonders for my profitability. I went from making average net $200 per day to over $500 per day overnight. I'm a very short term scalper. For others who are too cautious to make the jump, give it a try, you might be very surprised. Now I'm mulling over going to a 1000.
