Its all in the basics

According to the work energy theorem : $\large \dfrac12 m \cdot (\overline{v})^2 = \dfrac12 k \cdot x^2$ where, $k$ is the spring constant and $x$ is the maximum compression in the spring. $\overline{v}$ is the initial velocity of the block. At maximum compression state the velocity of the block becomes zero . So, on solving the above for $x$ we will get : $\large \boxed{x = v \sqrt{\dfrac{m}{k}}}$ Coming, to the next part of the question, as the surface is friction-less you may think that the block will reach with same velocity. It is true, but the thing you should consider is that, only the magnitude of velocity is same whereas the direction is opposite . So, the block will return with $- \overline{v}$ velocity which is not same as $\overline{v}$ .