yea there are several formulas that you will need to be familiar with in order to really dial in the motor size.. my personal tendencies are to wanna go too big, which means i have monster power, but it allways seems to end up hurting my overall performance and pocketbook in then end. screw type, size, moment of inertia of the screw, moving load and its inherant inertia, friction, drive type all play a role in it.. also something to consider, gearing, driver abilities and specs, powersupply specs and your budget all play a roll...
if your going to be using leadscrews moving a heavy gantry then your gonna need more torque, but if your using ballscrews and moving a light load, then you can go smaller..
having a monster motor, and not enough driver and or powersupply to drive it is wasted money as well... lots and lots of things go into the overall equasion...
Grizzly X3, CNC Fusion Ballscrew kit, 3 500oz-in bipolar steppers, 3 203v Gecko's, Linear power supply from Hubbard CNC, Mach 3, BOBcad Pro Art V22, Rhino.