A few points as to the sizing of motors and microstepping. This is a bit rambling but here is a description of the factors involved in the calculations.
There are no set rules or formula that you can choose the "perfect solution." The reason being that *it depends*. When you set up a system, you have to look at how you are connecting things mechanically. You are connecting a rotational movement to a linear movement. They mesh together different ways. All of them will create some backlash because they have to move against each other relatively freely. The tighter you couple them, the less backlash will be present but also you are increasing friction as you tighten them together. You need stronger components and more power to overcome this friction. Let's deal with them individually.
Stronger components.
As you get into larger and larger motors, the frame gets heavier, the shaft gets thicker and the coils get larger. This is pretty straight forward. There are some drawbacks with getting bigger motors. One is that they are heavier. Part of the load you are moving (depending on configuration) is the motor itself, say for a z-axis on a mill. The heavier weight means you have to have more power to move it but more importantly, you have to overcome the inertia of the extra mass to start and stop it. This requires more power. The larger coils also use more power (and weigh more) and make the cogging stronger so you get a more pronounced step in the motion rather than smooth motion. You can smooth part of it out with microstepping but you are also pushing and pulling at the same time which means that you don't have as much power available in the direction you want to move. So you get a bigger motor to overcome the drag of microstepping. See where this is going? Same thing happens when you beef up the drive components. Get a stronger leadscrew that is bigger and heavier, you have more inertia to start it and stop it. Needs more power.
Getting more power.
Ok, we have different ways to get more power. One is from pure mechanical advantage. As Archimedes was famous for saying, "Give me a long enough lever and I can move the Earth." Your options are most commonly a screw or a gear train. The finer the pitch of the leadscrew or the larger the gear ratio, the more power you can deliver to move something. Of course, there is no free lunch. The greater the mechanical advantage, the more rotation you have to put in to get the same movement. This is fine if you don't care about how fast you move but generally it is an important part of the overall decision. You want to be able to move fast enough to effectively cut things. Move too slow and you are not getting enough chip load and the friction will kill your cutters. Move too fast and you can't get chips to clear fast enough and you put too high of a chip load on the cutter and it breaks. Beyond that, you have to look at the differences in the motor types. Stepper motors are wonderful at slow speed. They deliver a huge amount of torque for their size. As they move faster though, they lose that power. On the other end, other types of motors such as servos, have more power at a higher speed but lose power as you slow down. You can also increase the power by putting more electricity through the motor. You can increase the voltage or the amperage to get more power.
Now, here are all the things that bite you in the rear.
Lets say you have a leadscrew. You want to move fast, the leadscrew has to turn faster or the pitch of the leadscrew has to give more motion for each rotation. Move too fast, and the screw can start whipping around and at the least create vibration and heat from friction on the nuts. At the worst, it can rip your drive system right out of the machine. Increase the pitch so it doesn't have to turn as fast, and you lose mechanical advantage, it has more torque on the screw which can bend it and make whipping worse, and there is more pressure on the nuts. Go with a gear train and each component of the train has it's own backlash that adds up to the overall backlash. Each component also has it's own inertia that will decrease the effective power transmission. Finer pitch on the gear train will give smoother motion but more friction. Coarser will give rougher motion. Go with stepper motors to get more low speed torque or servos to get more high speed torque. Gear up a stepper to get more speed, you get more cogging and rougher finishes. Gear down a servo to get the low end torque and you can't move as fast overall. Increase the voltage, you need to specify electrical components that have higher voltage ratings. Increase the amperage, you need to increase wire sizes and dissipate more heat.
What you really end up doing.
You can't just plug the numbers into a program and get the sizing you need for a system. All this can get pretty theoretical and looks like you need a degree in Electrical Engineering and Mechanical Engineering but the answer is that a lot comes by trial and error. You look at other systems and what they used and how well it worked. You also make decisions based on what you are going to be cutting, how much mass you are moving around, the power of the cutter, and how fast you need the system to move. Most of the factors can be accounted for. Backlash can be compensated for in software and it can be minimized by adjustments to the relationship of the components. You can control microstepping and go with fine steps when you want precision and use single steps for fast motion. You can do prediction of movement to slow motion down gradually instead of slamming on the brakes. Same for acceleration.
