For gallium nitride (GaN) high electron mobility transistors (HEMTs) distributed amplifiers (DAs), the relationship between gain and device size is analyzed in this letter when the total gate periphery is a reasonable and constant value. The analysis reveals that gain is a function of device size as the independent variable. With Lagrange analysis, a reasonable design for device size can be obtained to maximize the gain. Subsequently, a novel design method is proposed based on the theoretical derivation, aiming to enhance the gain performance of DAs. To validate the proposed design method, a broadband two-stage high-gain 10-W capacitively coupled DA has been implemented using the WIN 0.25- $\mu \text{m}$ GaN HEMTs process. The measured results of this nonuniform distributed amplifier (NDPA) show a small signal gain of 25 dB, an output power of 10–11 W, and an average power added efficiency (PAE) of more than 25% in the operating frequency range of 2–18 GHz. In addition, the designed NDPA monolithic microwave integrated circuit (MMIC) occupies a chip area of $3600\times 5000\,\,\mu \text{m}^{2}$ .