The standard trade size: 1 lot equals 100,000 units of the base currency.
The lot is the unit of trade size. A standard lot is 100,000 units of the base currency; a mini lot is 10,000 (0.10); a micro lot is 1,000 (0.01).
Choosing the right lot is the most direct way to control risk: the same price move produces twice the profit or loss if you double the lot. That is why it should be calculated from the risk you accept and your stop loss distance.