Sakes are usually served in small cups so that the drink can be finished quickly before the change in temperature alters the flavours. In Japanese culture, it is polite to pour drinks for others therefore a small cup allows constant refilling (by the way, its impolite to reject a refill, so never say no!).
As a general rule of thumb, sakes with subtle flavours should be sipped from a smaller sake cup made of a lighter material, such as glass. Sakes with stronger flavours would taste better when sipped from a wider rim cup which gives a longer and sweeter finish.
The most commonly used sake vessel is the ochoko, a small sake cup with a capacity of 18ml to 90ml. A well-known ochoko is the kikichoko, a cup with double blue rings at the bottom for drinkers to gauge the clarity of sakes from the contrast of the blue and white. The ochoko is always accompanied by a pitcher called the tokuri. To maintain a consistent temperature, the tokuri can be placed in a hot or ice bath. A slightly bigger sake cup called the guinomi can also be used in place of the ochoko. In the past, sake drinkers focused more on the taste rather than the nose of the sake. As such, smaller cups, which enhances the rich umami flavours, were used.
In recent years, it has become more popular to sip sakes in wine glasses in order to appreciate the aromas better. Wine glasses or wide rim sake cups are suitable for fragrant sakes like Daiginjos. As for sparkling sakes, champagne flutes can be used to showcase the bubbles.
However, there is no hard and fast rule on which cup to use. Always choose one that you fancy - a pretty cup makes a delightful drink.