\[S_{кр} = 8\pi\]
\[S_{кр} = \pi r^{2} = 8\pi\]
\[r^{2} = 8\ \]
\[r = 2\sqrt{2}.\]
\[a_{кв} = 2r = 2 \cdot 2\sqrt{2} = 4\sqrt{2}.\]
\[Ответ:4\sqrt{2}.\]