It would appear that 'jo' was originally the measurement unit of body length based on the length of adult male ('jobu' (丈夫) originally meant a man whose height was 1 jo [1.8 meters those days], whence it came to mean fully grown man).
「丈」は元々は成人男性の身長を基準とした身体尺であったと考えられる（「丈夫」は元々は身長1丈の男の意で、そこから一人前の男の意となった）。