The number of shares that are being offered for purchase at the bid price, often expressed in terms of hundreds of shares. Some traders try to use the bid size and ask size to measure impending short term upward or downward pressure on the stock's price. This can work for stocks on exchanges such as NYSE and AMEX, but is far less useful on Nasdaq, which has market makers ready to buy and sell shares, rather than specialists who balance books of buy and sell orders.