Answer
$2.6\;\rm cm$
Work Step by Step
This problem is similar to an aperture diffraction one. The lens here is the aperture and the diameter of the central maximum is the spot size.
We know that the minimum spot size to which a lens of diameter $D$ can focus light is limited by diffraction is given by
$$w_{\rm min}=\dfrac{2.44\lambda f}{D}$$
Solving for $D$; where $f=L$
$$D=\dfrac{2.44\lambda L}{w_{\rm min}}$$
Plug the known;
$$D=\dfrac{2.44 (1.06\times 10^{-6})(5\times 10^{-2})}{(5\times 10^{-6})}=\bf 0.026\;\rm m$$
$$D=\color{red}{\bf 2.6}\;\rm cm$$
