Length is the Metric of one Dimension of Space. The metric of space itself is volume, or (length)3. Length is commonly considered to be one of the Fundamental dimensions, meaning that it cannot be defined in terms of other dimensions.
Length is not an intrinsic property of anything, however, in that two observers can measure the same "thing" (i.e. distance between events, length of a board) and come up with a different answer. This strange property of space is explained by Einstein's special theory of relativity.