The vertical cocurrent downward air-water two-phase flow was studied under adiabatic condition. In view of studying the effect of flow area on interfacial structure and regime transition, round tubes of 25.4-mm ID and 50.8-mm ID were employed as test sections. The flow regime map was constructed for each test section by employing a newer approach. Unlike the conventional flow visualization method, the present approach minimizes the subjective judgment in determining the flow regimes. It was found that the flow regime in the co-current downward flow strongly depend on the channel size. The local two-phase flow parameters were acquired by the multi-sensor miniaturized conductivity probe in bubbly flow. They include: the local time-averaged void fraction, interfacial area concentration, bubble velocity and bubble Sauter mean diameter. The area-averaged data acquired by the impedance void meter were analyzed by the drift flux model. Three different distributions parameters were developed for the different ranges of non-dimensional superficial velocity, defined by the ration of total superficial velocity to the drift velocity. The new correlations can be applied to a co-current downward two-phase flow in a wide range of flow regime spanning from bubbly to annular flow.