Let $S$ and $S^{\prime}$ be the foci of an ellipse and $B$ be any one of the extremities of its minor axis. If $\Delta S^{\prime} B S$ is a right angled triangle with right angle at $B$ and area $\left(\Delta S^{\prime} B S\right)=8$ sq. units, then the length of a latus rectum of the ellipse is