what is the Relation of Motor Starting capacitor with Motor speed?More 2 questions in Desc
My answer to this other question provides a diagram and explanation of the why and how a capacitor in series with one winding affects the speed of a single-phase motor.

Increasing the capacitor value causes the current to increase in the winding that it is connected to. The maximum value of the capacitor is thus determined by the heating effects of increasing the current. The amount of speed increase is limited by the motor's synchronous speed, the speed of the magnetic field rotating in the motor. The synchronous speed is determined only by the number of poles created by the windings and the frequency of the applied power.

Selecting the capacitor value is part of designing the motor. There is no easy way to determine an appropriate range of values.