High-specific-power electrical machines are a key enabling technology of electric aircraft propulsion. The machine specific power (power-To-weight ratio) is usually constrained by the thermal and mechanical properties of material. To maximize machine specific power, electrical, thermal and mechanical capabilities have to be considered together. In this paper, an efficient multi-physics model of a high-frequency air-core Halbach-Array PM motor, has been developed for the use of a large-scale genetic-Algorithm-based optimization. The detailed electromagnetic (EM) and thermal models, and finite-element-Analysis (FEA) validation, are presented and discussed. Effects of key machine sizing parameters, such as speed, rotor volume, magnetic loading, and electric loading, in the multi-disciplines and objectives optimization, will be presented and discussed.