. 1.What is the word to describe something in the middle? Is it long or short? It is middle-sized (?). -- I'm not sure what you are looking for-- an adjective for an item of intermediate size or position? Here are some: middle-sized, intermediate, central, moderate, medial.
2. What do you say the word 'super' is - a prefix? Is that why it is hyphenated?-- Yes, it