The best way that I've always known if I'm in the right position size to account size is to be worried enough, yet calm or bored enough, to have a healthy zen like focus. If you are too worried you will get shaken out of trades prematurely and you will be looking for every little reason to exit your trade. If you are too calm and bored it can detract you from keeping a responsible eye on the position and it can also mean you are not utilizing your money line to its fullest potential. So which ever position size puts you in an alert, focused, state of mind. This does take practice to find the right emotional "zone" but it makes a huge difference on your trading. If you are in the right state of mind you can really see the market and its future direction MUCH more clearly. Also, I realize that some traders are numbers people, (I have a friend like this and he likes to argue with me over this from time to time), and they prefer to use exact numbers, (percentages, volatility, past price action, etc.), to gauge their position size. I have always been more of a discretionary, eye ball it and watch my emotions, kind of guy. Queuing off of my emotions is actually part of my methodology now and I am always surprised how much more my subconscious or "sixth sense" seems to know compared to what I consciously believe to see. Each trader needs to find what suits him/her best but emotional awareness is key either way in my opinion.