Size will depend on the weight you are trying to move and how fast you want to move it.
I found this formula to estimate linear force developed using a screw / motor combination:
1. Convert ballscrew lead to equivalent pully radius:
example:
Lead =0.200
0.200 / (2pi) = 0.200 / 6.283 = 0.0318" (equliv. radius pully)
2. Convert motor torque into linear force:
example:
Torque = 100 Oz.-In.
100oz-in / .0318 in = 3144 oz. (linear force)
3144 / 16 = 196 lb (linear force)
196 * 0.8 (efficency for ballscrew) = 157 lb linear force.
If this formula is correct, a 100oz-in motor and a 0.200 screw can push 157 pounds force.
The force you need to push depends on weight of the gantry, friction of the bearings, and cutting forces. (not to mention acceleration).
Hope this helps.
Bill