Short answers, pulled from the story.
Length is a measure of distance, the extent between two points in physical space. In the International System of Quantities it is a quantity with the dimension of distance, and most systems of measurement choose a base unit of length from which all other units are derived.
The metre, with the symbol m, is the base unit of length in the International System of Units. It is now defined in terms of the speed of light, which is about 300 million metres per second.
Length is the most extended dimension of a fixed object, while height is vertical length measured from a base. Width and breadth usually refer to a shorter dimension than length, and depth is used for the measure of a third dimension.
Length measures one spatial dimension, area measures two dimensions and equals length squared, and volume measures three dimensions and equals length cubed. The area of a rectangle is length times width, and the volume of a rectangular box is length times height times depth.
Common metric units include the metre, millimetre, centimetre, and kilometre, while U.S. customary, English, or imperial units include the inch, foot, yard, and mile, plus the nautical mile in navigation. Astronomy uses the astronomical unit, the light-year, and the parsec, while nuclear physics uses much smaller units such as the fermi.
Under special relativity, length is no longer constant across all reference frames. A ruler one metre long in one frame will not be one metre long in a frame moving relative to it, so an object's length depends on the speed of the observer.
In computer science, the length of a string is the number of characters it contains, and the length of a sequential data structure such as a dynamic array is the number of elements it holds. In graph theory, the length of a cycle, path, or walk in an unweighted graph is the number of edges it uses, while in a weighted graph it is the sum of the edge weights.