Surface tension and the length delta (distance between the Gibbs surface of tension R-s and the equimolar surface R-e) of simple liquid droplet (Lennard-Jones and Yukawa) are computed over a wide range of droplet sizes up to about 4 x 10(6) molecules. The study is based on the Gibbs theory of capillarity combined with the density-functional approach to gas-liquid nucleation. Since this method provides behavior of the surface tension fully consistent with the tension of the planner surface, the constant in Tolman's equation delta(infinity) can be determined unequivocally from the asymptotic behavior of sigma(s). Comparison of the tension given by Tolman's equation against the result of exact thermodynamic relations reveals that Tolman's equation is valid only when the droplet holds more than 10(6) molecules for the simple fluid systems near their triple points, in contrast to the conventional wisdom that Tolman's equation may be applicable down to droplets holding a few hundreds of molecules.