How big should be a piece of neutron star to reach Earth's mass? Imagine a neutron star could be broke into pieces.

How big would be the piece matching Earth's mass?
The question has no answer. The minimum mass for stable neutron matter is about 0.1 solar masses (see my answer here) and so is much more massive than the Earth.

As neutron stars become less massive they also become less dense, so your question is unanswerable at that level too, since we cannot put a figure on the density of something that cannot even exist in principle.

If your question is merely what radius is a spherical object that has a similar  density to a typical neutron star, can you not do
$$\frac{4\pi r^3}{3} \rho = M_{\rm Earth}$$
to find $r$?