长 (traditional 長) is a picture of a person with long, flowing hair — long hair was the clearest everyday image of 'long'. From hair it generalized to anything long, and to length of time.
长 has two readings worth knowing. As cháng it means 'long': 很长 (very long), 长城 (the Great Wall — 'long wall'!). As zhǎng it means 'to grow' or 'senior/chief': 长大 (to grow up), 校长 (school principal, 'school chief'), 长大 with 大. Same character, two lives — the long thing, and the act of lengthening/growing. China's 长城 (Great Wall) is the most famous 长 of all.
Long, flowing hair — LONG. (Read zhǎng, it's 'to grow'.)
长城 (the Great Wall) literally means 'Long Wall'. Read zhǎng, 长 means 'grow / chief': 长大 (grow up), 校长 (principal). One character, two readings.