In the present study we investigate the size and volume distribution of n-decane clusters in a brine saturated porous glass sample by means of micro-CT imaging with pore-scale resolution. Cluster size distribution functions and cluster volumes were obtained by image analysis for a range of injected pore volumes under both imbibition and drainage conditions, and with a field of view larger than the porosity-based representative elementary volume (REV). n-decane clusters were found to be distributed in sizes greater than that of the REV, with the largest one containing between 65 and 99% of the entire non-wetting phase. The results indicate that the two-phase REV may be substantially larger than for single-phase flow and larger than the attainable field of view.