Understanding the Standard Contract Size
When trading a currency pair, the standard contract size is called a “lot”. A lot can have different values for different currency pairs. At present, the major currency pairs have the lot value equal to 100,000 units. But 100,000 of what? When we buy, we give money for goods. When we sell, we get money for goods. But with currencies, we buy and sell simultaneously since we get one currency for another one. This is why it is common practice to consider that we get the first currency of the pair (base currency) when we buy and give, respectively, the second currency (quote currency).
A standard lot for USDCAD is $100,000 US dollars. If we buy one lot of USDCAD, it means that we get $100,000 US dollars and give a number of Canadian dollars equal to 100,000 times the actual exchange rate. If the current exchange rate of USDCAD is 1.1, then buying one lot of USDCAD means that we get $100,000 US dollars and give $110,000 Canadian dollars. The other way round if we sell one lot of USDCAD at the same exchange rate, it means that we give $100,000 US dollars and get $110,000 Canadian dollars.
Most of the forex brokers offer also the possibility to trade mini lots, which are one tenth of a standard lot (for example 10,000 units instead of 100,000 units).
Here you can find the lot and mini lot definition for the major currency pairs: