The distance is a singular distance. Five foot, three inch, &c. The unit used to measure the distance is what is being pluralised. The nail has a single length, this length is not three units. The measurement of the nail is three inches. A five feet wall would be a wall of five levels each a foot in height. In the same way a five men side of would consist of a team of five groups of men of unknown size (but greater than one), a five man side consists of one side of five singular men. Completely logical and straight forward when you realise that a team of several people is one team and a length of several units of measurement is one length.