This paper deals with determination of the size of the representative volume element (RVE) of an unsaturated sand specimen and its evolution during wetting. An imbibition test was conducted in an oedometer cell while stepwise reducing the capillary pressure by using the negative water column technique. X-ray tomography scans were acquired at each equilibrium state and three-dimensional images of a partial volume of the specimen were processed. Characterisation of the sample's homogeneity was based on the asymptotic behaviour method for three quantities of interest: the air fraction and the median diameters of both the air and the water-grain phases obtained from Matheron's granulometry. It is shown that the RVE size depends on the choice of the quantity of interest and evolves with the saturation state.