Order size, in trading, is the quantity that an order asks to buy or sell, stated as a number of shares, contracts or units of currency. It is one of the basic fields of every order, with the instrument, the side and the price condition. Exchanges set a minimum size and a trading increment, called the lot size, and they cap the largest order they will accept as a guard against a fat finger error.
What matters in practice is size relative to the market. An order that is small beside the quantity available at the best quote trades with little effect. One that is large beside the average daily trading volume pushes the price as it executes, a cost called market impact. Large orders are therefore split into smaller pieces over time or partly hidden with an iceberg order. Order size is distinct from position size, the total holding that a series of orders builds.