  Many-body localization in a disordered system of interacting spins coupled by
the long-range interaction $1/R^{\alpha}$ is investigated combining analytical
theory considering resonant interactions and a finite size scaling of exact
numerical solutions with a number of spins $N$. The numerical results for a
one-dimensional system are consistent with the general expectations of
analytical theory for $d$-dimensional system including the absence of
localization in the infinite system at $\alpha<2d$ and a universal scaling of a
critical energy disordering $W_{c} \propto N^{\frac{2d-\alpha}{d}}$. %The
finite size effect on the interaction stimulated delocalization of energy in
the ensemble of interacting two level systems in amorphous solids at low
temperature is discussed.
