Position sizing depends on volatility, volume and confidence. One symbol I track regularly I would use all 100K without concern. The volatility is low, the volume supports that amount, and I'm confident in my system. Conversely, consider shorting Best Buy last week based on negative news reports. The volatility is higher because news reports are less predictable than other technical or fundamental triggers; the volume would make it tough to get more than 50K in quickly; and using news reports as a trigger lessen my confidence in the trade. So in the 2nd example I would have used 50K instead of 100K. Last of all, it depends on experience. Until someone can trade 10K at a time with a high percentage of successful trades they shouldn't go to 25K. Once they can do 25K, then move to 50K, etc. Most traders don't follow this system, and that has a lot to do with why the failure rate is > 90% in this business.