Atmospheric-pressure RF plasma jets based on the parallel-plate COST-Jet reference plasma source geometry Golda J et al 2016 J. Phys. D: Appl. Phys. 49 084003) are investigated in He/O2 mixtures under different driving voltage waveforms. The effects of geometric modifications, specifically electrode widening and outlet scaling on power deposition and reactive-species transport are examined. These modifications are motivated by the limited material-processing area of the reference jet, whose nozzle cross section is only 1 mm x 1 mm. We first examine modified jet configurations with electrode widths of 1, 5, and 10 mm, and then scale up the 10 mm-wide jet by implementing different outlet structures, i.e., arrays of holes and slot patterns on one electrode to enlarge the effective treatment area. Electrical measurements show that Peak-type tailored voltage waveform excitation provides the highest power absorption at a given voltage amplitude and that the absorbed power increases nearly linearly with electrode width. When the absorbed power is appropriately scaled with the electrode width, the helium metastable densities measured by tunable diode laser absorption spectroscopy (TDLAS) remain nearly independent of the electrode width. For the geometrically modified electrodes, however, at the same absorbed power, the maximum helium metastable density decreases as the outlet distribution area increases. Two-dimensional fluid simulations capture the features of the measured helium metastable density distributions and further provide atomic oxygen density distributions characterized by a gradual downstream decay along the gas flow direction in the effluent. These results provide guidelines for the design of scalable and energy-efficient plasma jet sources.