Power performance results at 4 GHz are summarised for GaAs m.e.s.f.e.t.s ranging in size from 4 to 16 mm gate periphery. A double-chip 16 mm unit operated at 24 V source-drain bias produced 13.5 W with 3 dB gain and 10.7 W with 8.1 dB gain. Although lack of perfect power and gain scaling is observed, the degradation in output power of the 16 mm devices was only 1 dB compared to the smaller devices.