In Commercial work it's easy and I can not imagine it is much different in residential.
Drop a disconnect in the machine room for motor power, from disconnect into the elevator MCC.
Drop a disconnect in the machine room for car lighting power, in my area the elevator guys usually want a fused disconnect that takes a 15 amp fuse, from disconnect into the elevator MCC.
Elevator installer takes it from there.
As far as feeder size they will tell you clearly what they need, at least that's been my experience.
Don't forget outlets and lighting in machine room and Pit if it has one.
Hope this is helpful
Construction & Maintenance Electrician