for example an AC shaded pole fan motor 1550 rpm, 120v, 1.2A, not sure if this is enough info.i need to know approximately how much volts does it require for it to start spinning slightly.and what kind of motor specs should i be looking for if i want one that starts spinning at a low voltage? with the motor's length be between 2-4 inches.
That is not something that you can find out without testing the motor. Even then the voltage will vary a lot with slight changes in what is being driven, the temperature etc. AC motors are not usually designed for operation more than about 10 percent above and below the design voltage. A shaded pole motor driving a fan will behave more predictably than other types of AC motors and loads, but it probably won't behave very well below about half of rated voltage. If you want a motor that will work predictably over a wide voltage range, you need a DC motor.
