Start with the last question first and work backwards.
The proper way to choose a motor, is to set performance goals for your machine, and choose a motor and drive system to meet those goals. Motor manufacturers have motor sizing software, where you enter parameters and it tells you what motor to use. Or, do your own calculations to see how much torque you want at a given rpm, and choose a motor that will give you what you need. You'll usually want to try to achieve a certain acceleration rate, as higher acceleration is usually more desirable than using a low acceleration to achieve higher speeds.
You choose a motor based on the torque curve, which tells you how much torque a motor has at any given rpm.
Smaller frame motors tend to have higher maximum rpm's, which give them a flatter torque curve. Larger motors have steep torque curves, with very little torque at higher speeds.
Generally, the lower the motors inductance, the more torque it will have at higher speeds.
Nema 17 size motors really don't have enough torque for anything but very small machines. Generally, Nema 23 motors tend to be used if the required torque is up to 400oz-in or so. Above that, a low inductance Nema 34 will generally give better performance.