Day traders shouldn't risk more than 1% of their account on a single trade. If your forex day trading account is $1,000, then the most you'll want to risk on a trade is $10. If your account is $10,000, risk $100 per trade. Even great traders have strings of losses; by keeping the risk on each trade small, even a losing streak won't significantly deplete capital. Risk is determined by the difference between your entry price and the price of your stop-loss order, multiplied by the position size and the pip value (discussed in the scenarios below).
The need to exchange currencies is the primary reason why the forex market is the largest, most liquid financial market in the world. It dwarfs other markets in size, even the stock market, with an average traded value of around U.S. $2,000 billion per day. (The total volume changes all the time, but as of August 2012, the Bank for International Settlements (BIS) reported that the forex market traded in excess of U.S. $4.9 trillion per day.)