Bid Size

A bid for a stock (or any other tradable item) consists of two parts: a price and a size. The size signifies the amount of the stock a person is looking to buy. The price signifies what they are willing to pay (See: Bid Price).

So if you were looking to invest in Apple, you might make a bid of 200 shares at $189 per share. Now, you are waiting to get matched with an ask, i.e someone selling Apple stock who can fill your order of 200 shares at the price you set. (See: Bid and Asked)