  We report on the size dependence of the surface tension of a free surface of
an isotropic fluid. The size dependence of the surface tension is evaluated
based on the Gibbs-Tolman-Koenig-Buff equation for positive and negative values
of curvatures and the Tolman lengths. For all combinations of positive and
negative signs of curvature and the Tolman length, we succeed to have a
continuous function, avoiding the existing discontinuity at zero curvature
(flat interfaces). As an example, a water droplet in the thermodynamical
equilibrium with the vapor is analyzed in detail. The size dependence of the
surface tension and the Tolman length are evaluated with the use of
experimental data of the International Association for the Properties of Water
and Steam. The evaluated Tolman length of our approach is in good agreement
with molecular dynamics and experimental data
