Sake is sold in various sizes. However, the size is depended on the container. Sake sold at stores can be bottle, carton or can. The largest size of carton is 3 liters and the smallest is 180 ml. Normal sizes are 2, 1.8 and 900 ml. The size of the can is usually 180 ml, but some brand can be available in bigger size, around 300 ml.
On the other hand, the traditional size of bottle is 1,8039 liter, which is 1 sho in Japan. Sho is a unit of Japanese volume based on Japanese units of measurement called Shakkanho. 1 sho is 10 go.
Shakkanho was introduced from China but Japan introduced metric system in 19th century and shakkanho was banned to use for trade and evidence in 1958. Therefore 1 sho is not wriiten on the bottle but it is very common to use 1 sho bottle at liquor shop. Therefore soft drinks and alcohols originally from out of Japan use metric system but sake still have a habit to use the shakkanho.
However 1 sho is too big to keep or too heavy to carry or to consume. Therefore 1 sho bottles are not favorite for consumers and the retailers. Therefore instead of 1 sho bottle, 4 go; 720 ml bottle is much popular now. Therefore sake sold at duty free stores at airports are normally 720 ml bottle.
By the way, sake at restaurants are served by volume of 1 go or 2 go. 1 go is 180 ml. If you order 2 go, it usually comes in a ceramic container caller tokkuri.