In this section, 'Ken' does not represent the length of the measurement system, it is an architectural term for representing the number of intervals between two pillars; 'five Ken' refers to five hashirama (an interval of a space between two pillars) in six lines of pillars.
ここでいう「間」は長さの単位ではなく柱間の数を表す建築用語であり、「5間」とは柱が1列に6本並び、柱間が5つあるという意味である。