The number of orders in the order book of a security that come up immediately after the auction. Each exchange supports a maximum order book size on which the sizes of various asks and bids depend for trading.

If a huge order is placed on the order book (away from market prices), then a level of support might be building up. If traders continuously target that order trying to take it down but the size remains unchanged, it probably indicates that a lot more size is still there and the price evolution will be expected to to slow down. An if the size repeatedly gets knocked down, it means that once the large order is digested, the price trend will continue unabated. This situation is is known as a “decrementing bid/ offer” trade and is often noticeable in the case of trending stocks.