The term 'MOST EXPANSIVE' can be solved by the word 'WIDEST', which is a six-letter word. The term 'MOST EXPANSIVE' means having a large size or scope, and the word 'WIDEST' is used to describe something that has the greatest distance from side to side. This term is often used to describe physical objects, such as a room, a lake, or a mountain range. It can also be used to describe abstract concepts, such as a range of knowledge or a variety of experiences.