Most of the major motor / rail / screw suppliers provide sizing software on their web sites to do the calculations for you based on supplied information of your sysytem. Do a search and try some of them as they all do the same but the input interfaces are different. The result
can be used as a guide for intial selecton of components. I say guide because it provides a theoretical value and one still needs to
use some common sense on how conservative they want to be on sizing.
There is a practical way.
Once you build your system, ADJUST / LUBRICATE IT, but before you purchase motors, find the the torque to just move the axis by measuring it. Just use a torque wrench (or an equivilant like hung weights) to find the value. Then double or triple the value. Be consrvative
as this allows for acceleration at higher motor rpm values ( remember that a steppers torque decreases as rpm increases ) and a servo is sized differently.
There is nothing wrong with comparing equivilant size equipment to get a quick ballpark figure. But do make note of YOUR desired
operating conditions.
Designing a system can get complex quickly and one needs to understand what they are doing.
So, you intial question is a loaded one, and hope replied information helps you out.
RICH