13 cm
Let length = \( l \), width = \( w \).
Given: \( l \cdot w = 63 \), \( 2(l + w) = 32 \Rightarrow l + w = 16 \).
Solve: \( w = 16 - l \), so \( l (16 - l) = 63 \).
\[ l^2 - 16l + 63 = 0 \] \[ l = \frac{16 \pm \sqrt{256 - 252}}{2} = \frac{16 \pm 2}{2} = 9, 7 \] Length = 9 cm, width = 7 cm.
Thus, the answer is 9 cm.
For any natural number $k$, let $a_k = 3^k$. The smallest natural number $m$ for which \[ (a_1)^1 \times (a_2)^2 \times \dots \times (a_{20})^{20} \;<\; a_{21} \times a_{22} \times \dots \times a_{20+m} \] is: