  We assume that our universe originated from highly excited and interacting
strings with coupling constant g_s = {\cal O} (1). Fluctuations of spacetime
geometry are large in such strings and the physics dictating the emergence of a
final spacetime configuration is not known. We propose that, nevertheless, it
is determined by an entropic principle that the final spacetime configuration
must have maximum entropy for a given amount of energy. This principle implies,
under some assumptions, that the spacetime configuration that emerges finally
is a (3 + 1) -- dimensional FRW universe filled with w = 1 perfect fluid and
with 6 -- dimensional compact space of size l_s; in particular, the number of
large spacetime dimensions is d = 3 + 1. Such an universe may evolve
subsequently into our universe, perhaps as in Banks -- Fischler scenario.
