The word "arm length" refers to the measurement of the distance between the fingertips of one arm to the fingertips of the other arm. The antonyms of "arm length" are words that describe either a shorter or longer distance between the arms. One antonym could be "shorter reach," which means a shorter distance between the arms. Another antonym could be "longer span," which means a greater distance between the arms. Other antonyms could include "close proximity," "narrow width," "short extension," "restricted range," "diminished distance," or "constricted span." These antonyms help to describe a range of distances between the arms, from the very short to the very long.