For there are three things in every intellectual concept: the depth, length and width. The width is the aspect of the explanations of the concept to every side (from every angle) with many particular details as in the width of a river and not just the ideas as they are as in a narrow river.
And Now Chapter 1.03
The length of a subject is the intense descent of the mind necessary to enclothe a concept into various analogies, until the concept is brought within the grasp of even a small child etc. Like a river that flows forward extending over a great length. So that, just like a river flows downward carrying water from high in the mountains to low lying valleys and plains and the higher the mountain the longer its descent; so too the length of an idea is the measure of the amount of simplification needed to make a concept easy to understand.
Some concepts are very simple and can be readily understood by even a small child, while others are very far from the grasp of a child being naturally lofty and theoretical and so it is more difficult to explain simply. The "distance" between the natural understanding of the concept and the understanding of a child is the length of the concept.
(As is explained at length in the Chassidic manuscripts on the subject of the length of the skins used to cover the Tabernacle. What it basically explains there is that length implies one thing that undergoes many changes, yet retains its identity. For example, a long trip passes through a lot of different scenery and many stops on the way, but it all is really for one purpose, to move from point A to point B. All of the points in the middle don't change the theme of the trip, they only make it longer. Similarly when we say that an invention has come a long way, such as the development of the airplane since the Wright brothers' historical flight, we mean to say that it has developed tremendously, changing drastically. Yet, we can only say this about something that remains essentially the same. Like the airplane, whose basic concept has not changed, it is still a flying machine with wings. On the other hand we would not usually describe the development of the airplane from the bicycle as having come a long way, even though the Wright brothers used their knowledge of bicycles to help design their plane, because it is not one thing, the entire essence changed. So that just like physically we only describe something as long when remains essentially the same thing while undergoing many changes, so too a concept is described as long if is explained in a very different simpler way while remaining essentially the same. So that if Einstein were to explain his theory to a small child we could say that the theory went a long way from the understanding of Einstein to that of the child.)