Ask size is the number of shares, contracts or units offered for sale at the current ask price, the lowest price at which sellers are willing to trade. It appears next to the ask in a quote and is the selling-side counterpart of bid size. A quote of 50.10 × 800 on the offer means 800 units can be bought at 50.10. In some markets the figure is shown in round lots of 100 shares.
The figure tells a buyer how much can be bought at once without paying more. A market order larger than the ask size takes the whole quantity and then fills the rest at higher prices further up the order book. Traders compare bid and ask size as a rough guide to short-term pressure, but the displayed number is not the full picture: it leaves out hidden and iceberg orders, and quotes can be cancelled in an instant.