Non-integer slots do not matter. If you give me 54 slots, I infer that it must be a two-layer winding pattern. The electrical angle of the slot spacing is 26.67 degrees. You can set the span of the winding first. 27 slots are a cycle, and half a motor (generally 2 poles) can be designed. Non-integer slots are subdivided to the smallest integer pole. 54 and 8 can only be divided into 27 and 4, so we should design four poles.) This semi-motor has four poles with nine windings in each phase, occupying 18 slot. This can be 6 slots with a span of 160 degrees. Or that span of 7 slot is 186.67 degrees. The specific harmonic performance should be really designed before calculation.

The question you asked about the rotor size is too extensive and complicated. According to my personal experience, the stator size basically depends on heat dissipation. The size of the rotor mainly depends on whether the starting torque is large (two layers, high cost) and high efficiency. But the specific shape parameters are really not so uniform. As long as FEA is used up, the effect is ok, and what you can do is almost the same. The rest can only be compared.