In this research, the influence of channel size on co-current downward Taylor flow in a square channel is studied numerically by a volume-of-fluid method. Three different hydraulic diameters are considered, namely 0.5 mm, 1 mm and 2 mm. The physical properties of the two-phase system correspond to squalane ( liquid) and nitrogen ( gas). The simulations employ the unit cell concept where the ratio between unit cell length and hydraulic diameter is fixed to four and the gas holdup is 40%. For these conditions and each hydraulic diameter, a series of simulations with different flow rates is performed. The resulting values of the capillary and Reynolds number are in the ranges 0.15-0.5 and 2-17, respectively. The numerical results show that the appropriately non-dimensionalized bubble velocity, bubble diameter and volumetric interfacial area all scale with the capillary number, while the influence of inertia and buoyancy is small, though tiny effects can be recognized.