It seems as if Allen has confirmed that stanc can now be built with optimization, which reduces the size on disk considerably.
There is some question as to whether Python can specify
-std=c++0x when using g++-4.6 and
-std=c++11 when using g++-4.9. This can be done with R via an environmental variable since users of R 3.2.x are still on g++-4.6 while users of R 3.3.x are on g++-4.9. So, it seems as if we were to require C++11 for Stan, we would be limited to the C++11 features that were incompletely supported by g++-4.6 until April at least when R 3.4.x will be released, R 3.3.x will be considered stable, and R 3.2.x will be considered old stable. So, there is the question of whether there is enough in g++-4.6 to make it worthwhile
If we were to require C++11, it would cutoff anyone using g++-4.5 or lower, which is only prevalent on older Linux servers. Of course, at the moment g++-4.4 does not work even without C++11 due to things not being defined in the std library
./stan/math/rev/mat/functor/cvodes_utils.hpp:26: error: ‘runtime_error’ is not a member of ‘std’
./stan/math/prim/mat/fun/round.hpp:19: error: ‘std::round’ has not been declared
but we might be able to work around those.