  We present a finite-size scaling for both interaction and disorder strengths
in the critical regime of the many-body localization (MBL) transition for a
spin-1/2 XXZ spin chain with a random field by studying level statistics. We
show how the dynamical transition from the thermal to MBL phase depends on
interaction together with disorder by evaluating the ratio of adjacent level
spacings, and thus, extend previous studies in which interaction coupling is
fixed. We introduce an extra critical exponent in order to describe the
nontrivial interaction dependence of the MBL transition. It is characterized by
the ratio of the disorder strength to the power of the interaction coupling
with respect to the extra critical exponent and not by the simple ratio between
them.
