The total number of securities, belonging to a specific issue, that all buyers offer to purchase at the best bid. Differently stated, it is the number of round lots available on the buy side of the market.

A typical bid size is quoted in multiples of 100 (a round lot represents 100 securities). Therefore, a bid size of 4 equals 400 securities offered in a given market. A bid size of 100 equals 10,000 securities offered, and so on. Bid sizes are an important indicator of the demand and liquidity of a security at a given time.

The bid size and its counterpart, the ask size, are key for determination of the possibility to set up a profitable trade.