Determining what size of rice cooker you need is the first thing to consider when shopping for a machine. When measuring rice, Japanese people use a unit of measurement called gou (合), roughly 180ml. Gou is part of the shakkanhou (尺貫法) system of measurement, a traditional system that is no longer commonly used, but maintains its legacy as the standard for measuring both rice and sake. When you see the term “cup” in a rice cooker's description, it is likely 1 gou, not a metric cup (250ml) typically used in cooking and baking.
(Also, please note that 1 cup in modern Japan is equal to only 200ml, so please sure to adjust your recipes while cooking in Japan, and likewise if working off a Japanese recipe in the west.)
The shakkanhou system works like this: