Let $\overrightarrow {\bf{a}} = \hat i + 2\hat j + 3\hat k,\overrightarrow {\bf{b}} = 3\hat i + \hat j - \hat k$ and $\vec c$ be three vectors such that $\vec c$ is coplanar with $\overrightarrow {\bf{a}} $ and $\overrightarrow {\bf{b}} $ . If the vector $\vec c$ is perpendicular to $\vec b$ and $\vec a \cdot \vec c = 5$, then $|\vec c|$ is equal to