To keep it simple:
Bigger pulley on motor say 50mm, smaller on screw say 25mm (2:1) will half speed and double torque.
Bigger on screw 50mm, smaller on motor 25mm (1:2) will double speed and half torque.
So using same 50mm pulley in the example:
50/25 changes by factor of 2
50/10 would change by factor of 5
50/5 would change by factor of 10
I don't know if the ratio's I put in are opposite way around but you get the idea.