This work addresses the scalability of power performance of AlGaN/GaN MODFETs with large gate periphery, as necessary for microwave power devices. High-frequency large-signal characteristics of AlGaN/GaN MODFETs have been studied for devices with gate widths from 0.2 to 1 mm. 1-dB gain compression occurred at input power levels varying from -1 to +10dBm as the gate width increased, while gain remained almost constant at approx. 17dB. Output power density was maximum (1.3W/mm) for devices with 0.6mm gates and maximum output power (29.9dBm) occurred in devices with 1mm gates, while power-added-efficiency remained almost constant at approx. 30%.