Normal market size is the minimum number of securities for which a market maker is obliged to quote firm bidand ask prices. Of course, in a quote driven market, market makers cannot be expected to offer quotes that are firm up to an unlimited size. However, they must provide sufficient liquidity for investors to be able to transact reasonable quantities of a security at the quoted price. This is what constitutes normal market size.
On the London Stock Exchange, normal market size (NMS) for each security is calculated quarterly and is based on 2.5 per cent of the security's average daily turnover in the preceding year. Market makers are often prepared to quote firm prices for volumes larger than the NMS but must quote prices for volumes of at least NMS. You can read more about this process here.
If a company “Midco” has an NMS of 1,000, a market maker must be prepared to quote firm prices for volumes of at least that size. The market maker may go higher though, for example he may quote a size of 3,000 offer and 3,000 bid.
In this scenario your broker, on your behalf, should be able to buy or sell up to 3,000 shares in Midco via that market maker at the prices quoted by that market maker.
The market maker's quote will show on your broker's screen as 'Midco at 105 - 110 (3,000 x 3,000). This means the market maker is prepared to sell up to 3,000 shares at 110p or buy up to 3,000 shares at 105p.
If you wanted to buy or sell more than 3,000 shares, this may be possible, but you may have to pay rather more than 110p to get the shares, or accept rather less than 105p to sell the shares. Alternatively, breaking your transaction up into smaller trades may allow you to transact in higher volumes at the desired price.
Some key points to bear in mind: