The ask size refers to the number of shares or units of a financial asset that sellers are willing to sell at the ask price. It represents the quantity available for sale at the lowest price (the ask price) in the market at a given time.

Key points about ask size:

1. Quantity for Sale: It shows how many shares, contracts, or units are available at the current ask price. For example, if the ask price is $50, the ask size might be 500 shares, meaning sellers are willing to sell up to 500 shares at $50.
2. Market Depth: The ask size helps investors understand the depth of the market or how much volume is available at a particular price level. If the ask size is large, it may suggest that sellers are abundant at that price.
3. Part of the Order Book: The ask size is typically displayed alongside the bid size (the number of units buyers are willing to purchase at the bid price) in the market order book. The bid and ask sizes give insight into supply and demand dynamics.
4. Impact on Trade Execution: If a buyer places an order larger than the ask size, they may have to buy the remaining shares at higher ask prices. For instance, if the ask size is 500 shares at $50 and a buyer wants 1,000 shares, they may need to purchase the additional 500 shares at a higher ask price.

Understanding the ask size and the ask price helps investors gauge a security's liquidity and potential price movement when making trades.