KanjiJLPT N2
Length

length, ten shaku, measure, Mr., Ms., height, stature, all (one has), only, that's all, merely

Mnemonic

They reject anyone who can't reach the one mark carved high on the stone gate—the minimum length required to join the warrior clan. Children stretch desperately on tiptoe, terrified of being the one they reject for falling short of the sacred length.
Additional thoughts
Picture a single notch carved deep into an ancient stone gate. A child stretching on tiptoe, desperate to reach it. That one line represents full stature—your complete length from sole to crown.
Quick recall
Reject anyone under the one mark—the minimum length to join the warriors.

Details

The keyword for 丈 is length. This kanji originally referred to a unit of measurement equal to approximately ten feet, used in ancient China and Japan to measure height or distance. It conveys the idea of measuring something's full extent or stature. By extension, it can also suggest robustness or strength, as in someone who has reached their full length or grown to their complete height.
On'yomi
じょう
Kun'yomi
たけ、だけ

Used in kanji