(EDIT: sorry I had my adblocker on and didnt see the help you already got, its off now, its only my first day at the site.)
The longer the arm is the bigger the motor has to be, but I havent actually got much experience, im pretty sure if you did the mathematics its not a difficult problem to get right first time on paper.
What I can actually help with tho, is that it doesnt matter even if you just use ordinary motors without step control, as long as youve got a h-bridge to go back and forth, you can add the encoder yourself, James Bruton (xrobots) on youtube makes his own from scratch encoders... you just need to switch from full left to full right, and thats all you need, when the arm hits the target itll oscillate to stay still.
Also, you can use the maxwell effect, and use capacitors as the actuator, and then these actually dont need feedback to hit definite positions.