If I asked you how much money you risk per trade, you would probably pull out a calculator and tell me what 2 % of your account balance is. (dailypriceaction.com)
For more information, see client money risk in Part 1. (moneysmart.gov.au)
There's a direct correlation between how much money you risk on a trade and how emotional you become about it. (learntotradethemarket.com)