If you choose to use the word length it should refer to the longest dimension of the rectangle. How far from end to end. So in shorthand notation.
Arc length AB m AB 360 2pr A r c l e n g t h A B m A B 360 2 p r.
The length of the adjacent side. The length of an arc is found by forming a ratio of the arc to the whole circle then multiplying it by the formula for circumference either 2pr 2 p r or pd p d like this. S i n 53 o p. We are given the hypotenuse and need to find the adjacent side.