The phrase "medium size" refers to something that is neither very big nor very small. It describes an object or an entity that falls in between the extremes of size. Full definition
Melt chocolate in medium sized bowl in the microwave. (iheartnaptime.net)
In medium size bowl combine the flour, baking soda, and salt. (houseofyumm.com)
Both parent breeds have an average life expectancy between 11 - 14 years, which is also fairly typical for medium sized dogs in general. (wileypup.com)