  We present computations of certain finite-size scaling functions and
universal amplitude ratios in the large-N limit of the CP^{N-1} field theory.
We pay particular attention to the uniform susceptibility, the spin stiffness
and the specific heat. Field theoretic arguments have shown that the
long-wavelength description of the phase transition between the Neel and
valence bond solid states in square lattice S=1/2 anti-ferromagnets is expected
to be the non-compact CP^1 field theory. We provide a detailed comparison
between our field theoretic calculations and quantum Monte Carlo data close to
the Neel -VBS transition on a S=1/2 square-lattice model with competing
four-spin interactions (the JQ model).
