This work studies the diffusion of Hydrogen (H) in bcc Fe, containing a high-angle symmetric tilt grain boundary (GB), as a function of both the temperature and the average grain size. For this purpose, we propose a micro- scopic effective model which includes diffusion in bulk and in the GB. The model distinguishes between diffusion along the GB, in parallel with the bulk, while diffusion through the GB is to be considered in series. The bounding and migration energies of the H interstitial sites are derived through an extensive study of H atoms dissolved in a high-angle symmetric tilt GB. This is undertaken in the framework of a set of classical interatomic potentials, and partially from Density Functional Theory (DFT) calculations, in order to check the consistency of equilibrium atomic structures. We find that preferential trapping sites for H in the GB delay the H migration, thus enhancing its solubility. The derived H diffusion coefficients are in agreement with experimental evidence, however various kinds of GBs are present in real samples. In addition, we see that at high temperature, H diffusion does not depend on the grain size, as similar results than in bulk are found. In contrast, at room temperatures (290 K) and nano-sized grains (100 nm) the effective diffusion can slow down up to two orders of magnitude.