An electron temperature and a volume-averaged plasma density are experimentally investigated for various argon gas pressure and rf power in permanent-magnets-expanding plasma sources with two different diameters of 6.6 cm and 13.3 cm for the purpose of performance improvement of a electrodeless, magnetically expanding plasma thruster. The results are compared with a global model using particle balance and power balance equations. The theoretical values are in fair agreement with the measured ones. The experimental and modeled results suggest that a ~50 percent increase in the thrust from the electron pressure can be achieved by the enlargement of the source diameter from 6.6 to 13.3 cm.