Bid size is the number of shares, contracts or units that buyers are prepared to purchase at the current bid price, the highest price any buyer is offering. It is shown next to the bid in a quote and is the buying-side counterpart of ask size. A quote of 500 × 50.00 on the bid means buyers stand ready to take 500 units at 50.00.
For a seller, bid size shows how much can be sold at once at the best price. A larger sell order sent at market uses up that quantity and fills the remainder at lower bids, a cost measured as slippage. A bid size much larger than the ask size is sometimes read as buying pressure. The reading is unreliable alone: displayed size excludes hidden orders, and bids can be withdrawn before they trade. The full order book gives a wider view.